Fix occasional build errors.

We were occasionally receiving non-fatal errors during the build, which
seems to be an issue with gradle-retrolambda in combination with the most
recent version of the android gradle plugin.

The issue is being tracked in the gradle-retrolambda project here:
https://github.com/evant/gradle-retrolambda/issues/27

In the mean time, this was suggested as a workaround.  We don't rely on
support-annotations directly (they're a transitive dependency), so I
believe it should be safe to exclude them for now.