Enable deprecation warnings

And fix usage of Class.newInstance(), deprecated since Java 9

Change-Id: I3bc4a4b2354cc978dc934c7b29cff7a87ab90bc9
diff --git a/build.gradle b/build.gradle
index 944a7f8..84bb03c 100644
--- a/build.gradle
+++ b/build.gradle
@@ -83,6 +83,10 @@
   mustRunAfter test
 }
 
+tasks.withType(JavaCompile) {
+  options.deprecation = true
+}
+
 tasks.withType(Test) {
   reports.html.destination = file("${reporting.baseDir}/${name}")
   testLogging {