Add build status badge

README.md

 This site hosts code for <a href="https://iterm2.com">iTerm2</a>.
+[![Build Status](https://travis-ci.org/gnachman/iTerm2.svg?branch=master)](https://travis-ci.org/gnachman/iTerm2)
 [![Coverage Status](https://coveralls.io/repos/github/gnachman/iTerm2/badge.svg?branch=master)](https://coveralls.io/github/gnachman/iTerm2?branch=master)
 <a href="https://iterm2.com/bugs">File a bug report here!</a> Issues are on <a href="https://gitlab.com/gnachman/iterm2/issues">Gitlab</a> because Github doesn't support issue attachments.