Trying out Travis-CI's new Xcode 6.1 image.

No need to merge... not yet, anyway.
While it's not necessary right now, it seems likely that it'll be necessary in the future... I figured I'd leave it in (both the `brew update` and the `if brew outdated | grep -qx xctool; then brew upgrade xctool; fi` that actually updates `xctool`) and just ensure that the output spew from `brew update` is silenced (and the update check on `xctool` does, in fact, do nothing on the `xcode61` image).