Reference gem ‘bcrypt’ instead of its old name ‘bcrypt-ruby’

`bundle install` resulted in this 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.

#######################################################
~~~

As far as I can tell, the two gems are identical except `bcrypt` has a slightly newer version, so there should be no compatibility problems from this switch.