Improve performance on enforce_available_locales!

Cache the available_locales in a local Set, so we can lookup, and check for inclusions faster.
[fixes #230]
Merge pull request #249 from arthurnn/available_locales_perf

Improve performance on enforce_available_locales!
Nodoc and document new internal method [ci skip]

Also improve docs on enforce available locale related methods.
Fix bundle check guard clause on run_all script

So that it does not need to run bundle install all the time.