Add PHPCS to travis and fix violations.

Some warnings are left, but those won't fail the build.
Since phpcs seems to have no opinion about the file name, I'd prefer to use one that's a bit more indicative of what it's used for… I've been trying to see if there's a good precedent for phpcs ruleset file names. I think the closest I've found is simply `phpcs.xml` In the project root.