Bump the webmock and crack gems

This implicitly picks up the newer crack gem (the 0.3.1 gem has a
security vulnerability[1])

[1]: https://support.cloud.engineyard.com/entries/22915701-january-14-2013-security-vulnerabilities-httparty-extlib-crack-nori-update-these-gems-immediately
Merge pull request #208 from alphagov/bump-webmock-and-crack-gems

Bump the webmock and crack gems