Bump gem version to 3.1.0

Per http://semver.org, since the last version, "new, backwards compatible functionality [has been] introduced to the public API", so it should be incremented on the minor version.

Since there was no comment on https://github.com/codahale/bcrypt-ruby/pull/67, I'm not sure if you agree with the restructuring there.  If you agree with that one (or just want to discuss it on that pull request), you can just address this one _after_ that one.  If you disagree with the restructuring, that's a-okay with me (just a matter of preference), but I'd still like the gem version bumped so I can start using this version without needing git refs.

Thanks!