Add dev-lib and use for dogfooding rulesets (WordPress-Core) in Travis CI builds

[westonruter]

Fixes #256.

[JDGrimes]

This appears to be failing on HHVM: https://travis-ci.org/WordPress-Coding-Standards/WordPress-Coding-Standards/jobs/143952804

develop doesn't currently fail on HHVM, so that would be a regression.

[westonruter]

Humm. It seems to be failing during the install step but it doesn't indicate what the failure is. I'm not sure actually if dev-lib has run on the HHVM environment yet.

[GaryJones]

Can you please summarise what this change is doing?

[westonruter]

It's just using wp-dev-lib's builtin PHP syntax check and PHPCS running. The key feature of wp-dev-lib here is the ability to restrict error reporting to the changes in a PR. However, with #590 it is not necessary since the whole codebase is now getting cleaned.

[ntwb]

develop doesn't currently fail on HHVM, so that would be a regression.

Things might be different after #595 which updates the HHVM job to use the latest HHVM

[JDGrimes]

@westonruter Feel free to open another PR if you want to still use the dev-lib for syntax checking, etc.