Travis: run the code style related and ruleset checks in separate stages

Travis now offers stages. Using stages we can:

Run the code style related checks before running any unit tests and stop the build early if any are detected.

While this does mean that the unit tests will run with a slight delay (the Sniff stage has to finish before they start), it also means that we:

Get code style errors reported earlier as it's been moved to be the first stage and the build will just stop if any are found.
We won't be wasting Travis's resources on builds which will have to be re-run anyway.
The output of the Travis jobs will be a lot easier to decipher as it only shows the output related to that particular stage.

Note that Allowed failures is no longer listed as a separate block in the Travis result overview, but is is respected.

For more discussion about this:

WPTT / WPThemeReview