add build option to override gradle version check

There was a bug in older versions of the Android gradle plugin that makes gradle think that v2.10+ is older than v2.2 [1]. It then dies with this error "Gradle version 2.2 is required. Current version is 2.10. If using the gradle wrapper, try editing the distributionUrl...". The workaround is to add System.properties['com.android.build.gradle.overrideVersionCheck'] = 'true' to the buildscript {} section in build.gradle.[2] We should probably have a build option to add this. I think it should default off. @mvdan you've done most of this kind of thing. What do you think?