Fail build on yard warnings

I discovered we were getting warnings from YARD from rspec-core and fixed them in rspec/rspec-core#1752.  It flew under the radar for a long time so I figured it's worth adding to our builds so that we know when it fails.

Also, in vcr/vcr#440, @mislav did a lot of amazing work to make the VCR builds better and I figured I'd borrow some of the travis stuff (folding and timings) to make for more easily viewable travis builds.