CI: Submit Travis results to codecov

[effigies]

This should help identify whether code that's dependent on dependency versions is being exercised.

[codecov-io]

Codecov Report

Merging #2574 into master will increase coverage by 0.42%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2574      +/-   ##
==========================================
+ Coverage   67.13%   67.56%   +0.42%     
==========================================
  Files         333      333              
  Lines       42547    42547              
  Branches     5266     5266              
==========================================
+ Hits        28566    28746     +180     
+ Misses      13279    13128     -151     
+ Partials      702      673      -29

Flag	Coverage Δ
#smoketests	50.7% <ø> (ø)	⬆️
#unittests	64.99% <ø> (+0.51%)	⬆️

Impacted Files	Coverage Δ
nipype/utils/docparse.py	53.04% <0%> (ø)	⬆️
nipype/interfaces/fsl/utils.py	70.62% <0%> (ø)	⬆️
nipype/algorithms/icc.py	59.21% <0%> (ø)	⬆️
nipype/interfaces/base/core.py	89.61% <0%> (+0.14%)	⬆️
nipype/interfaces/fsl/model.py	80.61% <0%> (+0.18%)	⬆️
nipype/interfaces/afni/preprocess.py	81.5% <0%> (+0.24%)	⬆️
nipype/interfaces/io.py	54.47% <0%> (+0.31%)	⬆️
nipype/interfaces/spm/preprocess.py	57.87% <0%> (+0.35%)	⬆️
nipype/interfaces/freesurfer/preprocess.py	66.27% <0%> (+0.39%)	⬆️
nipype/interfaces/spm/base.py	87.45% <0%> (+0.66%)	⬆️
... and 23 more

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