Use AWS-SDK instead of Fog

I've rewritten the cache script to use the official [aws-sdk](https://rubygems.org/gems/aws-sdk) gem instead of Fog. This reduces the number of dependencies required on install and thus speeds up the builds a little bit.

I also added a `.travis.yml` config file that uses the gem from git to “test” itself on Travis. Unfortunately, you'll have to edit it to include your own AWS_S3_BUCKET, `AWS_S3_KEY` and `AWS_S3_SECRET` if you want to keep using it (There shouldn't be too much traffic from the testing since the bundle archive is under thee megabytes).

And I also added a slightly more descriptive description to the Gemspec. Oh, and it would be nice if you added me to the authors arrays (name & email) in the gemspec, but I didn't want to arrogantly add myself…