Making Gruntfile just test PhantomJS when a PR

Because of a [limitation between SauceLabs and Travis](http://support.saucelabs.com/entries/25614798-How-can-we-set-up-an-open-source-account-that-runs-tests-on-people-s-pull-requests-) that doesn't allow credentials to be passed when it a external PR

From the link:

>We've got good and not so good news for you. The not so good news first:
As things stand right now denying forks access to secure environment variables was a conscious design choice. The problem behind this is that the travis folks want to prevent malicious behavior where one could use some sort of console.log / print statement to leak plain text user credentials to any type of service.
We've raised this with the Travis folks and one of their devs and we are currently working on a token/HMAC based authentication method for Sauce that'll enable third parties (Pull Requests) to safely utilize the upstreams Sauce account without leaking username and access key. I unfortunately do not have an ETA on this yet. However, the solution is in the pipeline and actively being worked on.