Add Travis CI config

Since there was no reaction in #475 for a while, I went ahead and had a look at that.

The good news is that Travis has added explicit support for Android, so the build script is now very simple. Listing the components in `.travis.yml` is not strictly needed (they are [installed by default](http://docs.travis-ci.com/user/languages/android/#Overview)), but I thought it might save some time in the future when the defaults are changed. I also updated build tools to the latest version and added a travis build status image to the readme.

You can have a look at a successful build at https://travis-ci.org/fwalch/TextSecure/builds/24788883.