Update PR template to warn against outdated change

Recently, we've been receiving a lot of PRs which fix bugs occurring
only on older versions of bpftrace (e.g. changing args.xxx to args->xxx
in the one-liners tutorial).

Add a warning to the GitHub PR template that warns people to check that
they implement and test the change against the last version of bpftrace.

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,6 +4,10 @@ Please provide a description of your change below this comment.
 Then please complete the checklist.
 
 Useful contribution guidelines and tips are in docs/developers.md.
+
+Warning: please make sure that you have implemented and tested your
+         change against the latest version of bpftrace (unless opening a
+         PR for a release branch).
 -->
 
 ##### Checklist