Update CI to Python 3.6

[fmassa]

Python 3.6 (and 3.7) are by far the most downloaded distributions in anaconda.

Let's update the CI to test on Python 3.6.

This will simplify some other tasks down the road, like #1039, as PyAV version in conda is very old for Python 3.5

Another option is to add another build environment keeping Python 3.5, but I'm not sure it's necessary.

[codecov-io]

Codecov Report

Merging #1044 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1044      +/-   ##
==========================================
- Coverage   63.84%   63.77%   -0.08%     
==========================================
  Files          66       66              
  Lines        5275     5275              
  Branches      793      793              
==========================================
- Hits         3368     3364       -4     
- Misses       1675     1678       +3     
- Partials      232      233       +1

Impacted Files	Coverage Δ
torchvision/transforms/functional.py	70.79% <0%> (-1.18%)	⬇️

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