add x-maturity

[davefol]

Relevant changes are

• README.md: update environment variable documentation
• icees_api/app.py: stick "x-maturity" into openapi_args
• icees_api/trapi.py: openschema servers set as attribute instead of in initializer

Non relevant changes

• requirements.sh: remove pinned pandas (had trouble building on my system)
• test.sh: use docker port forwarding for test redis instance

Is there a way to

• confirm that this setup shows x-maturity manually
• automate testing that x-maturity is present

[codecov[bot]]

Codecov Report

Merging #192 (0eae328) into master (19cb90a) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #192      +/-   ##
==========================================
+ Coverage   81.39%   81.43%   +0.03%     
==========================================
  Files          15       15              
  Lines        1172     1174       +2     
==========================================
+ Hits          954      956       +2     
  Misses        218      218              

Impacted Files	Coverage Δ
icees_api/app.py	92.85% <100.00%> (+0.08%)	:arrow_up:
icees_api/trapi.py	90.32% <100.00%> (+0.32%)	:arrow_up:

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