Use containerized builds on travis-ci

MSP-11928

Travis-CI added faster, containerized builds recently.  They can be enabled with `sudo: false`.  Once containerized builds are used, bundler caching, previously a private, paid repository feature, becomes available using `cache: bundler`.

# Verification Steps
- [x] Look at the [build log](https://travis-ci.org/rapid7/metasploit-model/jobs/45480997)
- [x] VERIFY containerized message in log: `This job is running on container-based infrastructure, which does not allow use of 'sudo', setuid and setguid executables.`
- [x] VERIFY bundler cache in log: `adding /home/travis/build/rapid7/metasploit-model/vendor/bundle to cache`