Removes 0.9 for travis

Builds on this repo are currently [failing](https://travis-ci.org/wearefractal/gulp-coffee/jobs/26170351) because the VM no longer has Node 0.9.

This PR removes the 0.9 requirement and adds the unstable tag mentioned in travis-ci/travis-ci#2252.

*EDIT*: apparently, `unstable` isn't supported yet.