Stop symlinking the Gemfile

Just have the file in the root as expected, and keep the others we need
under ci/.
Move all Gemfiles to "gemfiles" folder

This is a more commonly used name, according to the docs:
http://docs.travis-ci.com/user/languages/ruby/
Fix script that runs tests against all available Gemfiles

BUNDLE_GEMFILE needs the full path to work properly.

Load all custom Gemfiles rather than a fixed list, so adding/removing
files from the gemfiles/ folder just works.
Fix example locale file for benchmark with newer yml