Gradle, take 2

My build.gradle and .travis.yml are shorter than yours, but still pass. I didn't need to set sdk.dir in local.properties and I don't download the large sysimg-18 emulator images.

https://travis-ci.org/cpeterso/MozStumbler/builds/9570305

If we don't care about supporting Eclipse users, we can probably delete `bin/.gitignore`. We might be able to delete `libs/android-support-v4.jar` now because people can/should install it with the Android SDK updater.