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

Should close https://github.com/scipy/scipy/issues/2758 
(Travis?)

Replace x**1.5 with np.power(x, 1.5) in stats.distributions: Raising np.inf to a fractional power fails in python2.6.

This is pretty much a hotfix: addresses an immediate issue, ignores many similar similar operations in specific distributions (which need to be addressed/tested separately, I suppose)