Fix include dir ordering in scipy.special

When installing into a virtualenv with a recent version of numpy, if the system-wide python includes have super-old numpy headers, scipy.special wont build correctly (e.g. missing NPY_EULER).  Prioritizing the numpy include dirs fixes this.
Merge pull request #2459 from llchan/master

BLD: Fix include dir ordering in scipy.special
TST: special/mpmath: fix precision issues in tests

- add leeway to airy tests -- loss of precision in trig functions

- deal with rounding issues near zeros of beta and binom

- the accuracy of iv seems limited to ~ 1e-280 --- that's probably still OK, so add some leeway

- re-enable Bessel Y test, was fixed in 56a1cbd6c6b
ENH: LinearOperator .dot()
Merge pull request #2464 from argriffing/linear-operator-dot

ENH: sparse.linalg: add LinearOperator .dot()