BUG: fix failing tests in stats (gh-2758)

Replace x**1.5 with np.power(x, 1.5) in stats.distributions.
Raising np.inf to a fractional power fails in python2.6,
which triggers a bunch of fails in
https://github.com/scipy/scipy/issues/2758
BUG: np.inf**2 fails on python2.6; replace by np.inf*np.inf
Merge pull request #2760 from bburlo/issue2758

BUG: fix failing tests in stats (gh-2758)