Remove unsupported node version from ".travis.yml"

Travis CI never seems to have supported node 0.9.

Until recently, Travis CI silently ignored unsupported node versions and used the default version, as you can see in [this `gulp-util` build log (from April 5)](https://travis-ci.org/gulpjs/gulp-util/jobs/22359264).

These days, Travis CI will fail if unsupported node version is requested ([list of supported versions](http://docs.travis-ci.com/user/ci-environment/#Node.js-VM-images)). This pull request removes version `0.9` from `.travis.yml` to allow proper build success/error reporting.