Don't ask travis to update xctool anymore

Travis have upgraded to the latest xctool. Running `brew upgrade xctool` has a failure exit status if it is up to date (handy!). So let's err on the side of upgrade xctool when we start having problems with xctool.
Merge pull request #85 from Ashton-W/travis-brew-error

Fix: Don't ask travis to update xctool anymore