Make .gemspec simpler

* Remove tests related files from the gemspec so that a user installing
  the gem won't download them when doing `gem install rails_legit`.
i dont know about advantages. one thing which i can think of is: if there are some ignored files, or not checked in files in lib folder, they also get included when we do `Dir.glob`. with `git ls-files` its only the files which are checked in.