MAINT: update numpy version check and warning in ``scipy/__init__.py``.

Note that recommended version is changed from numpy 1.5.0 to 1.5.1.
1.5.0 had a distutils bug that caused scipy not to build on OS X, plus
some other fixes that may be relevant for scipy.
All binaries have been built against 1.5.1 for a long time.
MAINT: remove numpy.oldnumeric import.

Is removed in numpy 1.9.0, see https://github.com/numpy/numpy/pull/3638
Merge pull request #3177 from rgommers/version-warning

Update numpy version warning, remove oldnumeric import