Build docs in standard workflow, not with travis

[mih]

Also RTD PR build is turned on now.

[codecov]

Codecov Report

Merging #171 (941babf) into master (2f4dd4d) will decrease coverage by 0.37%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #171      +/-   ##
==========================================
- Coverage   86.19%   85.82%   -0.38%     
==========================================
  Files          17       18       +1     
  Lines         927      931       +4     
==========================================
  Hits          799      799              
- Misses        128      132       +4     

Impacted Files	Coverage Δ
datalad_container/_version.py	0.00% <0.00%> (ø)

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