Auto-value dependency drags dep into truth's downstream

Auto-value annotations dependency is specified as a default (compile) dependency, which in maven-land means needed at compile time and runtime. This means that it leaks the version truth uses to any downstream project that fails to specify it, possibly conflicting with newer usages. Since Truth only needs auto-value to build itself, we should specify the dep as provided so it's not included in the transitive closure of Truth.