Fix security vulnerability

As discussed in #8 Googles implementation of the In-app-billing sample has some serious security flaws. This commit applies the suggested fixes https://code.google.com/p/marketbilling/source/detail?r=7bc191a004483a1034b758e1df0bda062088d840.
Switch to generally available robolectric version 2.2 since there is no need to use the possibly unstable SNAPSHOT