CocoaPods caching

This builds on #229.
It would be great if this was a bit more flexible in terms of the location of the Podfile (and in turn, the location of the `Pods/` directory). Either it could be define in the .travis.yml file, or it could some how figure out where it is.

The `Podfile` is normally in the root directory for applications. However it's very rare that it is in the root directory for libraries. Here are some examples:

- [AFNetworking](https://github.com/AFNetworking/AFNetworking) - `Podfile` is in the `Tests` directory.
- [NSAttributedString+CCLFormat](https://github.com/cocodelabs/NSAttributedString-CCLFormat) - `Podfile` is in the `Tests` directory.
- [KFData](https://github.com/kylef/KFData) - `Podfile` is in the `Tests` directory.

The template for `pod lib create` also doesn't store the `Podfile` in the root directory. But instead inside the example project. https://github.com/CocoaPods/pod-template