NEP29: Bump minimum numpy to 1.13

[effigies]

As of January 16, the next minor release (3.1) is to require numpy 1.13.

TODO:

• Check for any code that branches on numpy < 1.13
• Titrate a minimum packaging while we're at it.

Ref #803.

[codecov]

Codecov Report

Merging #868 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #868      +/-   ##
==========================================
+ Coverage   89.66%   89.67%   +<.01%     
==========================================
  Files          96       96              
  Lines       12378    12378              
  Branches     2184     2184              
==========================================
+ Hits        11099    11100       +1     
  Misses        933      933              
+ Partials      346      345       -1

Impacted Files	Coverage Δ
nibabel/volumeutils.py	84.13% <0%> (+0.19%)	⬆️

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[effigies]

Looks like we can use any version of packaging. That's simple.