Set Java source version to 13.

Change-Id: Ic3b2f5332f7a00c4153b80fdd063e223367fef6f
diff --git a/build.gradle b/build.gradle
index b7f76f1..518e861 100644
--- a/build.gradle
+++ b/build.gradle
@@ -57,5 +57,8 @@
   }
 }
 
+sourceCompatibility = 13
+targetCompatibility = 13
+
 group 'eu.mulk'
 version '0.1.0-SNAPSHOT'