Bump bcrypt-ruby version to support Ruby 2.0 on Windows

bcrypt-ruby version 3.1.0 had a bug that caused it to not work on Windows at all — see https://github.com/codahale/bcrypt-ruby/issues/72

Version 3.1.1 fixed the bug, but limited bcrypt-ruby to only work on Ruby 1.9 on Windows — https://github.com/codahale/bcrypt-ruby/pull/73

Version 3.1.2 (the most current stable version) adds fat-binary support for Rubies 1.8, 1.9, and 2.0 for Windows, and works for 32 and 64 bit Windows — https://github.com/codahale/bcrypt-ruby/pull/78

This just updates to the version that should work on the most platforms.