Upgrade build tools to version 25.0.2

Build tools 25 is a requirement for newer Android Gradle plugins.

Change-Id: Ifb3400d2d22fdde900a7cfb85739a60b4a9a20e5
diff --git a/app/build.gradle b/app/build.gradle
index 6430dcb..1b3c151 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -2,7 +2,7 @@
 
 android {
     compileSdkVersion 23
-    buildToolsVersion '23.0.2'
+    buildToolsVersion '25.0.2'
 
     defaultConfig {
         applicationId 'com.google.reviewit'