Bumps rugged to version 0.18.0.b1, resolving GPG issues

Hey there!

I discovered another bug with GPG-signed commits while rolling out Gitlab CI at my workplace.

GPG signatures slip through the parsing and are displayed as part of the commit message:
![Bildschirmfoto 2013-04-20 um 14 57 51](https://f.cloud.github.com/assets/690188/405155/952ac608-a9bb-11e2-8257-5f73a07f9ed1.png)

This issue has been resolved in [libgit2/rugged #1255](https://github.com/libgit2/libgit2/pull/1255) three months ago and is included in the most recent pre-release version of the rugged gem:

![Bildschirmfoto 2013-04-20 um 15 12 15](https://f.cloud.github.com/assets/690188/405161/f796bca2-a9bb-11e2-8c04-0c8da8718b52.png)