Upgrade to protobuf gradle plugin 0.8.13

This version is needed to better support Gradle configuration
caching.
https://github.com/google/protobuf-gradle-plugin/releases/tag/v0.8.13

Test: ./gradlew assembleDebug
Change-Id: I1d7793e75f7d68493fa7e6adccad630a439f744c
diff --git a/build.gradle b/build.gradle
index 7f32c42..6d13dc2 100644
--- a/build.gradle
+++ b/build.gradle
@@ -21,7 +21,7 @@
 
 buildscript {
     dependencies {
-        classpath('gradle.plugin.com.google.protobuf:protobuf-gradle-plugin:0.8.8')
+        classpath('gradle.plugin.com.google.protobuf:protobuf-gradle-plugin:0.8.13')
         classpath('org.anarres.jarjar:jarjar-gradle:1.0.1')
     }
 }