Upgrade to Gradle 2.13 to avoid Robolectric/JaCoCo bug

[mkjensen]

This issue tracks upstream bugs gradle/gradle#575 (comment), jacoco/jacoco#288 (comment), robolectric/robolectric#2230 (comment).

Basically, a bug in Gradle/JaCoCo/Robolectric prevent code coverage from working. The current fix implemented in config/jacoco.gradle is to perform some hacks that enable the use of an old version of JaCoCo. These hacks should be removed ASAP.

[mkjensen]

Invalid since I have removed Robolectric in favor of classic instrumentation tests using Espresso in 3a273bd. I don't have much experience with Robolectric but at the moment I don't think it helps me as much (faster tests) as it slows me down because of issues I don't understand.