Add showing java version to prepare_release.sh

Patch by brandonwilliams; reviewed by edimitrova for CASSANDRA-19614
diff --git a/cassandra-release/prepare_release.sh b/cassandra-release/prepare_release.sh
index cff842e..f4638e1 100755
--- a/cassandra-release/prepare_release.sh
+++ b/cassandra-release/prepare_release.sh
@@ -166,6 +166,7 @@
     fi
     echo ""
     git show $head_commit
+    java -version
 
     echo "Is this what you want?"
     select yn in "Yes" "No"; do