Compile support-v4 with Java 7 in gradle

Fixes build

Change-Id: Idec75c0feb617b72af9b5ae0d35c8c84f87de1b6
diff --git a/v4/build.gradle b/v4/build.gradle
index 1cf63e8..d689a3f 100644
--- a/v4/build.gradle
+++ b/v4/build.gradle
@@ -88,6 +88,11 @@
         // TODO: fix errors and reenable.
         abortOnError false
     }
+
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_7
+        targetCompatibility JavaVersion.VERSION_1_7
+    }
 }
 
 android.libraryVariants.all { variant ->