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

enry.go

@@ -1,3 +1,16 @@
+/*
+	Package enry implements multiple strategies for programming language identification.
+
+	Identification is made based on file name and file content using a seriece
+	of strategies to narrow down possible option.
+	Each strategy is available as a separate API call, as well as a main enty point
+
+		GetLanguage(filename string, content []byte) (language string)
+
+	It is a port of the https://github.com/github/linguist from Ruby.
+	Upstream Linguist YAML files are used to generate datastructures for data
+	package.
+*/
 package enry // import "gopkg.in/src-d/enry.v1"
 
 //go:generate make code-generate