Update com.google.protobuf: 0.8.10 -> 0.8.12

The update reduces Gradle warnings
diff --git a/gradle.properties b/gradle.properties
index 8a90bdb..8f70cbb 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -38,7 +38,7 @@
 com.github.johnrengelman.shadow.version=5.1.0
 com.github.spotbugs.version=2.0.0
 com.github.vlsi.vlsi-release-plugins.version=1.60
-com.google.protobuf.version=0.8.10
+com.google.protobuf.version=0.8.12
 de.thetaphi.forbiddenapis.version=2.7
 org.jetbrains.gradle.plugin.idea-ext.version=0.5
 org.nosphere.apache.rat.version=0.5.3