improve rendering of travis-ci build status

* destretch by removing dimensions, also future proofs size changes
* forcibly move to next line
* uses css attr selector, other strategies may be better
Merge pull request #2318 from blissdev/gh-pages

Improve Travis-CI Badge Rendering in Docs
adding a class to the travis badge
Merge branch 'master' into gh-pages
Merge remote-tracking branch 'remotes/documentcloud-original/gh-pages' into gh-pages