Mark which version of HoTT/coq we use

Mark this in .travis.yml, and note it in README.markdown, as described
by https://github.com/HoTT/HoTT/pull/166#issuecomment-22460613.
Merge pull request #170 from JasonGross/travis-ci-commitish

Mark which version of HoTT/coq we use
Merge branch 'master' of https://github.com/HoTT/HoTT