[MRG+1] Skip test for OrthogonalMatchingPursuitCV if running on Travis

OrthogonalMatchingPursuitCV has been having heisen-failures on Travis for quite some time. 

See #3190 for more details.

I have been unable to replicate this test on a local box using varying Python, numpy, scipy, and BLAS implementations. I have also been unable to replicate this using a CoreOS VM on Rackspace using Docker (along with the above veriations), and severely limiting memory available to the container. One way to avoid this failure is simply skip this particular test on Travis, using a similar technique as #3333.