Add Sonatype OSSRH upload support, with nice POMs

Mockito was upgraded to latest to solve JavaDoc issue with
ArgumentsAreDifferent.

We don't yet publish any artifacts for grpc-compiler.
diff --git a/compiler/build.gradle b/compiler/build.gradle
index 47bcad4..1efbe83 100644
--- a/compiler/build.gradle
+++ b/compiler/build.gradle
@@ -78,4 +78,7 @@
   commandLine './src/test/run_nano_test.sh'
 }
 
-
+[
+    install.repositories.mavenInstaller,
+    uploadArchives.repositories.mavenDeployer,
+]*.addFilter('none') { artifact, file -> false }