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.

http://docs.scipy.org/doc/numpy/release.html#npy-relaxed-strides-checking
a small glitch: what does `pip install --upgrade` do if the requested version is *lower* than the installed one? I mean, when numpy 1.9.2 is out on PyPI, which one will this pick?