use Travis CI

ref.
  http://www.neilmiddleton.com/deploying-to-heroku-from-travis-ci/
  http://metabates.com/2012/10/23/deploying-to-heroku-from-travisci/
add a build status badge of Travis CI
execute "travis encrypt HEROKU_API_TOKEN=<SECRET> --add"
Merge branch 'travis-ci'