Add continuous integration (Fixes Issue #5)

Add continuous integration support per #5.

Takes advantage of the feedvalidator using UNIX exit statuses correctly (e.g. feedvalidator emits exit 0 with valid feed and non-zero status on a feed with issues) and TravisCI interpreting exit 0 as success and everything else as failure.
the generated feed_info.txt and zip are just used for this validation step, correct? They are also generated by https://github.com/codeforamerica/node-gtfs-zip-proxy and I want to make sure they don't get duplicated