Fix #496, Rate app link in overflow menu

The test is very minimalistic for a few reasons. On the emulator, there
is no Play Store App, on a device there probably is, but clicking the
menu would then leave Catroid. This would cause us to lose control over
the test, because we're not in Catroid anymore.

@RoxaneKoitz, please verify the wording. I implemented it using the 
imperative form *Rate us!* and *Bewerte uns!*.