Update to new Gradle for Android Studio 2.1

build.gradle

@@ -4,7 +4,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.0.0-beta6'
+        classpath 'com.android.tools.build:gradle:2.1.0-alpha4'
     }
 }