Add Travis CI config

Since there was no reaction in #475 for a while, I went ahead and had a look at that.

Travis has added explicit support for Android, so the build script is quite 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 the build tools to the latest version and added a travis build status image (will look like [![Build Status](https://travis-ci.org/fwalch/TextSecure.svg?branch=travis)](https://travis-ci.org/fwalch/TextSecure)) to the readme.

You can have a look at a successful build at https://travis-ci.org/fwalch/TextSecure/builds/24788883.