MNT/FIX: Bump minimum matplotlib to 1.5.2 to match available wheels

[effigies]

Per #886 (comment), ancient wheels are no longer being hosted for free. The only affected minimum dependency we have is matplotlib, which we can bump to 1.5 without likely affecting users.

The oldest wheel for Python 3.6 is 1.5.3, so I'm going with that to avoid doing this again in a couple months.

[codecov]

Codecov Report

Merging #887 into master will increase coverage by 1.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #887      +/-   ##
==========================================
+ Coverage   89.65%   90.73%   +1.08%     
==========================================
  Files          96       96              
  Lines       12418    13480    +1062     
  Branches     2187     2525     +338     
==========================================
+ Hits        11133    12231    +1098     
+ Misses        940      913      -27     
+ Partials      345      336       -9

Impacted Files	Coverage Δ
nibabel/pkg_info.py	81.48% <0%> (-3.14%)	⬇️
nibabel/keywordonly.py	100% <0%> (ø)	⬆️
nibabel/arrayproxy.py	100% <0%> (+0.74%)	⬆️
nibabel/dataobj_images.py	94.79% <0%> (+0.76%)	⬆️
nibabel/analyze.py	99.56% <0%> (+1.03%)	⬆️
nibabel/freesurfer/mghformat.py	96.89% <0%> (+1.34%)	⬆️
nibabel/cifti2/cifti2.py	98.14% <0%> (+1.46%)	⬆️
nibabel/brikhead.py	99.11% <0%> (+1.57%)	⬆️
nibabel/parrec.py	95.46% <0%> (+3.59%)	⬆️
nibabel/minc1.py	94.93% <0%> (+4.18%)	⬆️
... and 3 more

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[effigies]

I'm assuming this won't be very controversial, but reviews are welcome.