This adds a android-apk-build which runs on travis-ci. This is using a
quite ugly trick, building in a docker container, basically just to get
a newer cmake. The cmake in trusty is way to old to work with android
builds.
A good side-effect is that this is a complete copy-paste for anyone who
would like to build android-binaries them self on Linux. All the
uglyness is hidden away in a docker container.
Signed-off-by: Anton Lundin <glance@acc.umu.se>
