Attempt to solve unit tests failing due to pytest

[stevetorr]

Travis logs indicate that pytest is at too low a version (perhaps due to a change in some requirement by another library). I have fixed this by adding a minimum version to pytest in the requirements.txt.

[jonpvandermause]

Good catch, this seems to do the trick

[stevetorr]

Thanks! I'll pull this once the travis passes.

[codecov-commenter]

Codecov Report

Merging #186 into master will increase coverage by 0.22%.
The diff coverage is 83.92%.

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
+ Coverage   58.62%   58.84%   +0.22%     
==========================================
  Files          35       35              
  Lines        7523     7545      +22     
==========================================
+ Hits         4410     4440      +30     
+ Misses       3113     3105       -8     

Impacted Files	Coverage Δ
flare/gp.py	81.11% <83.92%> (+2.84%)	⬆️

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