Enable Java 8 features

https://developer.android.com/studio/write/java8-support.html

Change-Id: Ia974d88497c966af942339866f75dfd3c7d4d17c
diff --git a/app/build.gradle b/app/build.gradle
index c47ca4a..de674a8 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -15,8 +15,8 @@
         multiDexEnabled true
     }
     compileOptions {
-        sourceCompatibility JavaVersion.VERSION_1_7
-        targetCompatibility JavaVersion.VERSION_1_7
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
     }
     buildTypes {
         release {