TravisCI: Use flake8-rst-docstrings plugin

[peterjc]

This will put our docstrings though docutils to check they are valid as reStructuredText (RST), using my flake8 plugin:

• https://pypi.python.org/pypi/flake8-rst-docstrings
• https://github.com/peterjc/flake8-rst-docstrings

Note just because a docstring passes this validation does not mean it will render as expected.

Cross reference #1221

[codecov]

Codecov Report

Merging #1324 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1324      +/-   ##
==========================================
- Coverage   84.48%   84.45%   -0.04%     
==========================================
  Files         319      318       -1     
  Lines       48851    48779      -72     
==========================================
- Hits        41273    41196      -77     
- Misses       7578     7583       +5

Impacted Files	Coverage Δ
Bio/PDB/Model.py	73.33% <0%> (-22.13%)	⬇️
Bio/PDB/Residue.py	67.7% <0%> (-19.2%)	⬇️
Bio/PDB/Chain.py	78.72% <0%> (-13.59%)	⬇️
Bio/ExPASy/Enzyme.py	76.25% <0%> (-10.06%)	⬇️
Bio/PDB/Structure.py	92% <0%> (-3.66%)	⬇️
Bio/SeqRecord.py	93.51% <0%> (-0.82%)	⬇️
Bio/Seq.py	98.45% <0%> (-0.02%)	⬇️
Bio/PDB/PDBParser.py	88.13% <0%> (ø)	⬆️
Bio/SeqIO/__init__.py	84.43% <0%> (ø)	⬆️
Bio/KEGG/Gene/__init__.py
... and 7 more

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