Don't install ruby-debug if running the test suite on Travis, linecache19 is the main offender, very very slow. And do not install pg if Travis is bundling the gems, pg will be setup on Travis soon.
Cherrypick for 3-1-stable - adding w3c_validators gem
it's loading rbx-require-relative > 0.0.4
…dency of linecache which is a dependency of ruby-debug. However, the latest release of linecache (0.45) omits the dependency from its gemspec, causing the build to fail. I have emailed the maintainer so we should remove this when it's fixed upstream.