data race detected by CI

[emidoots]

https://travis-ci.org/sourcegraph/go-langserver/jobs/460093521

WARNING: DATA RACE
Write at 0x00c420318020 by goroutine 44:
  github.com/sourcegraph/go-langserver/langserver.updateCachedDiagnostics()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/diagnostics.go:104 +0x2ee
  github.com/sourcegraph/go-langserver/langserver.(*LangHandler).publishDiagnostics.func1()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/diagnostics.go:69 +0xac
  github.com/sourcegraph/go-langserver/langserver.(*diagnosticsCache).sync()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/diagnostics.go:50 +0xbf
  github.com/sourcegraph/go-langserver/langserver.(*LangHandler).publishDiagnostics()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/diagnostics.go:67 +0x272
  github.com/sourcegraph/go-langserver/langserver.(*LangHandler).typecheck()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/loader.go:77 +0x92f
Previous read at 0x00c420318020 by goroutine 48:
  github.com/sourcegraph/go-langserver/langserver.(*LangHandler).publishDiagnostics()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/diagnostics.go:81 +0x2ef
  github.com/sourcegraph/go-langserver/langserver.(*LangHandler).lint()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/lint.go:44 +0x1ff
  github.com/sourcegraph/go-langserver/langserver.(*LangHandler).lintPackage()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/lint.go:59 +0x57e
  github.com/sourcegraph/go-langserver/langserver.(*LangHandler).Handle.func5()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/handler.go:478 +0x12a
Goroutine 44 (running) created at:
  github.com/sourcegraph/go-langserver/langserver.(*LangHandler).Handle()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/handler.go:471 +0x601
  github.com/sourcegraph/go-langserver/langserver.(*LangHandler).handle()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/handler.go:154 +0x7f
  github.com/sourcegraph/go-langserver/langserver.(*LangHandler).(github.com/sourcegraph/go-langserver/langserver.handle)-fm()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/handler.go:30 +0x76
  github.com/sourcegraph/go-langserver/vendor/github.com/sourcegraph/jsonrpc2.(*HandlerWithErrorConfigurer).Handle()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/vendor/github.com/sourcegraph/jsonrpc2/handler_with_error.go:21 +0x97
  github.com/sourcegraph/go-langserver/langserver.lspHandler.Handle()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/handler.go:51 +0xc6
  github.com/sourcegraph/go-langserver/langserver.(*lspHandler).Handle()
      <autogenerated>:1 +0x90
  github.com/sourcegraph/go-langserver/vendor/github.com/sourcegraph/jsonrpc2.(*Conn).readMessages()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/vendor/github.com/sourcegraph/jsonrpc2/jsonrpc2.go:511 +0x131
Goroutine 48 (running) created at:
  github.com/sourcegraph/go-langserver/langserver.(*LangHandler).Handle()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/handler.go:475 +0x71a
  github.com/sourcegraph/go-langserver/langserver.(*LangHandler).handle()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/handler.go:154 +0x7f
  github.com/sourcegraph/go-langserver/langserver.(*LangHandler).(github.com/sourcegraph/go-langserver/langserver.handle)-fm()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/handler.go:30 +0x76
  github.com/sourcegraph/go-langserver/vendor/github.com/sourcegraph/jsonrpc2.(*HandlerWithErrorConfigurer).Handle()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/vendor/github.com/sourcegraph/jsonrpc2/handler_with_error.go:21 +0x97
  github.com/sourcegraph/go-langserver/langserver.lspHandler.Handle()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/langserver/handler.go:51 +0xc6
  github.com/sourcegraph/go-langserver/langserver.(*lspHandler).Handle()
      <autogenerated>:1 +0x90
  github.com/sourcegraph/go-langserver/vendor/github.com/sourcegraph/jsonrpc2.(*Conn).readMessages()
      /home/travis/gopath/src/github.com/sourcegraph/go-langserver/vendor/github.com/sourcegraph/jsonrpc2/jsonrpc2.go:511 +0x131
==================
--- FAIL: TestIntegration_FileSystem_Lint (0.06s)
	testing.go:730: race detected during execution of test

[berkant]

Is this happening indeterminedly?