build: update gradle

Change-Id: Ie9b2afb815355532d2c0df8b821100904da17288
diff --git a/ring-android/build.gradle b/ring-android/build.gradle
index 5671e6c..c89dacd 100644
--- a/ring-android/build.gradle
+++ b/ring-android/build.gradle
@@ -4,7 +4,7 @@
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.3.0'
+        classpath 'com.android.tools.build:gradle:3.3.1'
         classpath 'com.google.gms:google-services:4.2.0'
     }
 }