Run Gerrit batch init upon install

Change-Id: I21872a5a93bb69d52bd786b37b722183a6feada0
diff --git a/gerrit.rb b/gerrit.rb
index 5db92d2..39bbd97 100644
--- a/gerrit.rb
+++ b/gerrit.rb
@@ -21,5 +21,7 @@
     mkdir_p prefix/"plugins"
     (prefix/"plugins").install "plugins/uploadvalidator.jar"
     (prefix/"plugins").install "plugins/avatars-gravatar.jar"
+
+    system "java", "-jar", bin/"gerrit.war", "init", "--batch"
   end
 end