Changed promise to resolved on failing tests to not break sequence

First, fantastic job on this :-)

I had issues using grunt-buster with grunt-contrib-watch. I had no problems running them together, but when a test fails the "deferred.reject()" call causes the "sequence" in the buster.js task file to run the reject callback, not passing the two server variables needed to stop grunt-server and phantomJS. This causes the "stop()" function in buster.js to actually break since it tries to resolve "results[0]" and "results[1]" of an argument that is undefined.

All of this ends up stopping the grunt task whenever a test fails, not continuing with the watch task to rerun everything when you have fixed the test.

I tried a couple of different solutions, but ended up with the thought that if a test fails it does not mean that the sequence failed. It should still close down the servers.

Hopefully I got this right :-)