Use the new bcrypt gem, not bcrypt-ruby

See the change upstream at:

https://github.com/codahale/bcrypt-ruby/commit/273946f2ba549b9119868331388e8cf30d2af37c

Reported by @ZeroChaos

## Verification

- [x] Run `bundle install`
- [x]  See that the `bcrypt` gem is loaded (currently, version 3.1.7)
- [x] launch `./msfconsole`
- [x] See that the following error **does not** happen:

````
[-] WARNING! The following modules could not be loaded!
[-] 	/home/todb/git/rapid7/metasploit-framework/modules/auxiliary/admin/http/cfme_manageiq_evm_pass_reset.rb: LoadError cannot load such file -- bcrypt
````

While the gem name is changed to `bcrypt` the old filename is unchanged from `bcrypt`. So, it's a good change, but sadly means that lots of people will accidentally be stuck on the old version until they notice the version drift.