build: Fix gradle plugin version

Change-Id: Ic81a859af4356bec9d8d4e9688317feb53baac2f
diff --git a/build.gradle b/build.gradle
index e40b82e..b510b10 100644
--- a/build.gradle
+++ b/build.gradle
@@ -8,7 +8,7 @@
         }
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.0.0-beta5'
+        classpath 'com.android.tools.build:gradle:2.1.0'
         classpath 'net.named-data.playground:play-publisher:1.2.0-2-SNAPSHOT'
 
         // NOTE: Do not place your application dependencies here; they belong