BLD: use NPY_RELAXED_STRIDES_CHECKING for one TravisCI configuration

This affects TravisCI so check its output carefully before merging.  In fact TravisCI should segfault in ndimage if TravisCI does what I am intending, at least until something like https://github.com/scipy/scipy/pull/4218 has been merged.  This PR should close https://github.com/scipy/scipy/issues/4217.