Fix bug in rake task file exclusion
Fix bug in rake task file exclusion

The "exclude_js_files" option would only work if "js_files" was also
set, which does not seem to be the desired behavior. For instance, this
patch allows specifying a pattern to match and then using
"exclude_js_files" to exclude specific files matched by the pattern.

stereobooster/jshintrb,thoughtbot/jshintrb