Have Travis build us a dependency graph

This does most of #204 - I still want to, eventually, get a theorem/definition dependency graph using http://www-sop.inria.fr/lemme/coq-graphs/.  I'm going to push this once Travis finishes, because it barely touches the build system (it adds a target to the makefile) and mostly just touches the travis stuff.  The Haskell code could be prettier (I haven't done too much Haskell, so my patches are hacky), and I'm open to other color suggestions.

Here's a preview:
The core library:
![hottcore](https://f.cloud.github.com/assets/396076/2202617/75ab874a-9907-11e3-9490-c393db6d06c5.png)

With categories:
![hott](https://f.cloud.github.com/assets/396076/2202624/a015a146-9907-11e3-8728-67ff35d4877e.png)