refactoring: remove un-used code, add go doc, fix ci (#199)

Refactoring, consisting of
 - remove unused method `isAuxiliaryLanguage` and `FileCountList`
   in order to reduce public API surfaces (go/java)
 - add GoDoc to public APIs
 - ci: java profile use latest go src
  It also now mimics https://docs.travis-ci.com/user/languages/go/#go-import-path
  for non-go build image, as code relies on internal imports.

TEST PLAN:
 - make test

common.go

@@ -26,6 +26,7 @@ var DefaultStrategies = []Strategy{
 	GetLanguagesByClassifier,
 }
 
+// DefaultClassifier is a naive Bayes classifier based on Linguist samples.
 var DefaultClassifier Classifier = &classifier{
 	languagesLogProbabilities: data.LanguagesLogProbabilities,
 	tokensLogProbabilities:    data.TokensLogProbabilities,