Cache the cabal build

[gelisam]

The two stack builds take 2 minutes each on travis, while the cabal build takes 17! I'm using a sandbox, maybe we can cache its contents? I kind of like the fact that the cabal build checks whether hawk still builds fine with the latest version of everything though... maybe there's a way to detect if a new version of the dependencies have been released, and to clear the cache in that case?

[gelisam]

Now that the tests are also being run on Travis, the difference is more along the lines of 20 minutes vs 30 minutes, so the improvement would be smaller.

On my 5 year old laptop, it only takes 1 minute to build after a stack clean, and 1 minute to run the tests, so I guess Travis is using much older, slower machines. Which is fine with me, it's a free service after all.