Upgrade gradle to version 2.1.0 to benefit from instant run.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=121941168

build.gradle

@@ -20,7 +20,7 @@ buildscript {
         jcenter()
         jcenter()
     }
     }
     dependencies {
     dependencies {
-        classpath 'com.android.tools.build:gradle:1.5.0'
+        classpath 'com.android.tools.build:gradle:2.1.0'
         classpath 'com.novoda:bintray-release:0.3.4'
         classpath 'com.novoda:bintray-release:0.3.4'
     }
     }
 }
 }