`bcrypt-ruby` has changed its name to `bcrypt`.
https://github.com/codahale/bcrypt-ruby/blob/master/bcrypt.gemspec#L2

Here's the update warning:
```
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.

```