Run Coverity scan on Travis

This adds a build to the Travis matrix that generates a Coverity scan and submits it to be listed at https://scan.coverity.com/projects/639.

- [x] Generate Coverity scan results
- [x] Upload them to the scan site
- [x] Badge for readme
- [x] Restrict to `development`
- [x] ~~Write a modeling file to tune the results~~ (we'll address this later)