Fix "hash" Breakage

The `hash` method in Ruby is important and, unless overriding it is intentional and adheres to convention, it should not be overridden. This pull request fixes that following the interface of `bcrypt-ruby` by having the attribute be called `checksum`. Since the change to a _very_ public API is not backward compatible, I bumped the major version of the gem in a separate commit. (http://semver.org/)