Update manifest and gemspec

Without these changes we get this warning:

loofah at /Users/rafaelfranca/.rvm/gems/ruby-2.1.2/bundler/gems/loofah-9831fa472731 did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  [".gemtest"] are not files