Remove Python2, torch 0.4 and add torch1.7 CI support

[silvandeleemput]

Closes #58

This PR implements:

• Added torch 1.7 to tests, also an automatic latest version build
• Made the Dockerfile use torch 1.7
• Removed all Python2 tests from CI (support has been dropped, time to move on)
• Removed all torch 0.4 tests from CI
• Made test compatible with PyTorch 1.7 (stats.py::accuracy method fix)
• Adjusted the README.rst

[codecov[bot]]

Codecov Report

Merging #59 (a02aa79) into master (b1d4eff) will increase coverage by 0.17%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
+ Coverage   98.21%   98.38%   +0.17%     
==========================================
  Files          17       16       -1     
  Lines        1063     1053      -10     
==========================================
- Hits         1044     1036       -8     
+ Misses         19       17       -2     

Flag	Coverage Δ
backend	?
frontend	?
unittest	?

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Impacted Files	Coverage Δ
memcnn/models/revop.py	94.40% <100.00%> (-0.18%)	:arrow_down:
memcnn/utils/stats.py	100.00% <100.00%> (ø)

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