Pass `filePath` to compiler.shouldCompileFile() instead of `code`

[alreece45]

shouldCompileFile() is passed code, instead of filePath. The tests also suggest that the expected return value is filePath, rather than code. This commit corrects both to use filePath

Code loading is shifted so that it is only loaded when shouldCompileFile() returns true, rather than always.

[alreece45]

It looks like there was an issue with nvm on the Travis build, can we request a re-run?

nvm --version
0.33.2
157.22s$ if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update      ; fi
error: RPC failed; curl 56 SSLRead() return error -36
fatal: The remote end hung up unexpectedly
fatal: early EOF
fatal: index-pack failed
Error: Fetching /usr/local/Homebrew/Library/Taps/caskroom/homebrew-cask failed!
Updated 2 taps (homebrew/core, homebrew/versions).