migrate to java 8

Change-Id: I5bafd6e6b9d3b482ff18379043f6635f248c293c
diff --git a/ring-android/app/build.gradle b/ring-android/app/build.gradle
index 3cd53f6..6647b44 100644
--- a/ring-android/app/build.gradle
+++ b/ring-android/app/build.gradle
@@ -71,8 +71,8 @@
         release.setRoot('build-types/release')
     }
     compileOptions {
-        sourceCompatibility JavaVersion.VERSION_1_7
-        targetCompatibility JavaVersion.VERSION_1_7
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
     }
     buildTypes {
         release {