Mark TestUdfs.test_mem_limits to run serially

This was causing other tests to fail with process mem limit exceeded.

Change-Id: I1407b0896052aece691c681827994961b09d8103
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2001
Reviewed-by: Ishaan Joshi <ishaan@cloudera.com>
Tested-by: jenkins