build: bump gradle plugin

Change-Id: I04705bc9501b391f4ecc65c3d6271354366f28aa
Reviewed-by: Aline Bonnet <aline.bonnet@savoirfairelinux.com>

ring-android/build.gradle

@@ -4,7 +4,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.3.0'
+        classpath 'com.android.tools.build:gradle:2.3.1'
         classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'
 
         // NOTE: Do not place your application dependencies here; they belong