MAINT: Update numpy and Python requirements

[effigies]

Changes proposed in this pull request:

• Add python_requires metadata, matching the Python classifiers
• Require numpy >= 1.14, as indicated in TypeError problem from monoexponential.py #141

Closes #395.
Closes #396.

[jbteves]

LGTM, thanks @effigies ! Usually we squash and merge these, you may merge it when you're ready.

[effigies]

@jbteves Sorry, added a commit after your review.

[codecov]

Codecov Report

Merging #397 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #397      +/-   ##
==========================================
+ Coverage   81.26%   81.27%   +<.01%     
==========================================
  Files          41       41              
  Lines        2375     2376       +1     
==========================================
+ Hits         1930     1931       +1     
  Misses        445      445

Impacted Files	Coverage Δ
tedana/info.py	100% <100%> (ø)	⬆️

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[emdupre]

Thanks, @effigies and @jbteves !