Modernize cap_gun source

Hi there,

first of all I want to thank you for cap_gun! It's a very useful gem, and I am happy to have it available for the projects I am working on.

However, I felt that the code needed a refactoring, like using bundler, action mailer 3 (to not need a custom TLS hack), travis build and so on.

That's why I created a fork, and released it as a separate gem for now: xing-cap_gun. 
The source is here: https://github.com/xing/cap_gun

Please check the list of commits, maybe you find some changes useful enough to make it into the official version.

Thanks!
Sebastian