Sbtandroid 0.7, no instrumentation tests

This pull request update sbt-android-plugin 0.6.x to sbt-android 0.7, and disabled instrumentation tests (they are broken and not essential for LibGDX applications).

When sbt-android will be updated, we will think about one of two solutions:
- reenabling instrumentation tests
- using Robolectric instead

for now it is good to push this update out, because all essential stuff outside of testing works nicely.