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

java/src/main/java/tech/sourced/enry/Enry.java

@@ -9,16 +9,6 @@ public class Enry {
 
     private static final EnryLibrary nativeLib = EnryLibrary.INSTANCE;
 
-    /**
-     * Returns whether the given language is auxiliary or not.
-     *
-     * @param language name of the language, e.g. PHP, HTML, ...
-     * @return if it's an auxiliary language
-     */
-    public static synchronized boolean isAuxiliaryLanguage(String language) {
-        return toJavaBool(nativeLib.IsAuxiliaryLanguage(toGoString(language)));
-    }
-
     /**
      * Returns the language of the given file based on the filename and its
      * contents.