Run appropriate test strategy for Travis CI

For security reasons, Travis CI doesn't provide Sauce Labs credentials for pull requests, resulting in failures like this one: https://travis-ci.org/ExactTarget/fuelux/builds/6278737

Created 'grunt travisci' task to use Sauce Labs when credentials are available or fall back to PhantomJS otherwise.

When I submit this pull request, we'll be able to see whether it worked.