Pull jmh Gradle plugin version to top level (#526)

This better matches how we list versions for other plugins.
diff --git a/build.gradle b/build.gradle
index 245f10d..aad03ea 100644
--- a/build.gradle
+++ b/build.gradle
@@ -32,6 +32,7 @@
   id "com.github.johnrengelman.shadow" version "6.1.0" apply false
   id "com.github.nbaztec.coveralls-jacoco" version "1.2.5" apply false
   id "com.android.application" version "3.5.0" apply false
+  id "me.champeau.jmh" version "0.6.6" apply false
 }
 
 repositories {