Updated Android SDK to 23, gradle to 1.12

* updated Android SDK to 23
* updated gradle to 1.12
* changed filter name for 'sys-img-armeabi-v7a-android-19' (Maybe google changed name)
* updated README

I updated the box @ 1.0.4
https://app.wercker.com/#applications/53996c7711a7495563003b65/tab/details