[Travis CI] OS X build is failing

[tclemos]

$ make build-osx
rm -Rf ./builds/osx
CGO_ENABLED=1 CC="gcc" GOOS="darwin" GOARCH="amd64" go build -o ./builds/osx/flappy-gopher -tags static -ldflags "-s -w" ./cmd/flappy-gopher/main.go
# github.com/veandco/go-sdl2/sdl
/usr/bin/ld: -f may not be used without -shared
collect2: error: ld returned 1 exit status
makefile:28: recipe for target 'build-osx' failed
make: *** [build-osx] Error 2
The command "make build-osx" exited with 2.