Update runner.html

Updated the placement of the blanket script reference, and made the data-cover-only attribute more specific.

Coverage should work after this.

As for the messy header, that's a bug in the Blanket mocha adapter.  The adapter is causing the mocha runner to be run twice, which is causing a DOM artifact.  I'm going to update the mocha adapter in the blanket repo, and then you can `npm update` or manually copy the change into your repo.