TST: stats: guard test_moments with an env variable

Attempt to work around TravisCI 50 min timeout by fencing an expensive set of tests in stats by an env variable. 

Open questions: 
 * do we even want this [I think we do]
 * what sort of granularity of control do we want. At the moment I'm using the same env variable which is used in `test_sparsetools` --- IMO one env variable is enough.
 * do we want to expose this to other places in scipy. ATM it's in `lib._util`, but it can be moved closer to where it's used in stats. Alternatively, we may want to move https://github.com/scipy/scipy/blob/master/scipy/sparse/tests/test_sparsetools.py#L19 to `lib._util` as well.

For the moment though, I'd like to see whether this actually helps (TravisCI?).