Travis-CI integration

This pull request adds Travis-CI integration to the repository. On each push to each branch, Travis-CI will automatically run tests. Pull requests will now display the status of the build, and the badge in the README will display the status of the build for master.

Note that this travis-ci configuration will pull all frenetic-lang dependencies from github and build and install them from source, using the current HEAD. Right now the only dependency is frenetic-lang/ocaml-packet.