Upgrade gradle

Change-Id: I1a899e2e757c1266a60afdb4e43d9b2ab9805639
diff --git a/build.gradle b/build.gradle
index 9c1a198..73d0250 100644
--- a/build.gradle
+++ b/build.gradle
@@ -8,7 +8,7 @@
         }
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.1.2'
+        classpath 'com.android.tools.build:gradle:2.3.3'
         classpath 'net.named-data.playground:play-publisher:1.2.0-2-SNAPSHOT'
 
         // NOTE: Do not place your application dependencies here; they belong