Run ShellCheck on Travis

@charlierudolph The `.travis.yml` file was getting a bit voluptuous, so I extracted a dedicated shell script. This also has the advantage that we can lint it.

Travis uses Ubuntu 12.04, which has no shellcheck package, hence I have to compile it from source here. Andrew Mohrland helped me with the setup. 
I took them out and they broke the build, so this seems a generic Git property and not just Github related. Putting them back in.