BF: don't use dtype with ones/zeros_like

Use ones/zeros(arr.shape, dtype) instead - for compatibility with older
numpy - at least numpy 1.5.0 does not have this.
RF: bump minimum numpy version to 1.5
RF: up numpy and scipy dependencies as discussed
RF: drop scipy requirement to match travis-ci
Merge pull request #374 from matthew-brett/numpy-compat-fixes

MRG: bumpy numpy requirement to 1.5 / compat fixes