rename gem bcrypt-ruby to bcrypt

"bcrypt-ruby" changed its gem name to "bcrypt" about two months ago (https://github.com/codahale/bcrypt-ruby/pull/86). Now when I am running a vanilla rails bundle install command, I got following message:

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.

#######################################################

I think it's better to rename the gem file to the most updated one (i.e. bcrypt) used by bcrypt-ruby project people.