Rename bcrypt-ruby to bcrypt to avoid post-install warning

bcrypt-ruby tells us to do so after installing through bundler:

  Post-install message from bcrypt-ruby:

  The bcrypt-ruby gem has changed its name to just bcrypt.  Instead of
  installing `bcrypt-ruby`, you should install `bcrypt`.  Please update your
  dependencies accordingly.
Merge pull request #33 from brightin/rename-bcrypt-ruby

Rename bcrypt-ruby to bcrypt to avoid post-install warning