COMPAT: numpy compat with NaT != NaT, #12049
closes #12049 

cc @shoyer 

so just a minor change actually. So ok from me to leave numpy as is I think. (though you still might want to deprecate / do next release in any event)
 I think that people will be unhappy if numpy issues a release that breaks the latest release of pandas.... so I think numpy needs to wait either way.
 ok should be good now.

if its changed to a `DeprecationWarning` I _think_ we shouldn't show it, but will have to see
 LGTM
 now if Travis-CI would just work :< notice its been red all day 
