Downgrade scipy/numpy requirements. Fix #1450

[menshikh-iv]

We should to minimize numpy/scipy version
For test/build we need numpy/scipy wheels,
Minimal wheels version for linux from PyPI

*	numpy	scipy
2.7	1.6.0	0.9.0
3.5	1.9.0	0.16.0
3.6	1.11.3	0.18.1

[menshikh-iv]

We can use different versions of the dependencies depending on the python version, but It will bring pain and suffering.

As more people move to 3.6 from 2.7 + we want to build gensim wheels for all main python versions (2.7, 3.5 and 3.6), for this reasons I'll use versions that anywhere available (i.e. from 3.6).

You agree with me @piskvorky?

[piskvorky]

Up to you @menshikh-iv , I have no preference either way.