CI: Run stable tests weekly, pre-release tests nightly

[effigies]

Lost this when we moved from Travis.

[codecov]

Codecov Report

Merging #1025 (770113d) into master (a0d2534) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1025      +/-   ##
==========================================
+ Coverage   92.25%   92.26%   +0.01%     
==========================================
  Files         100      100              
  Lines       12185    12201      +16     
  Branches     2132     2134       +2     
==========================================
+ Hits        11241    11257      +16     
  Misses        616      616              
  Partials      328      328              

Impacted Files	Coverage Δ
nibabel/loadsave.py	93.84% <0.00%> (+0.04%)	⬆️
nibabel/volumeutils.py	93.52% <0.00%> (+0.05%)	⬆️
nibabel/openers.py	95.57% <0.00%> (+0.47%)	⬆️

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