Modify Version#payload to include alias to number called version to match Rubygem attributes

[zenspider]

I found it incredibly awkward that

https://rubygems.org/api/v1/gems/rubygems-update.json

returns an attribute called version, but

https://rubygems.org/api/v1/versions/rubygems-update.json

does not. This fixes that w/o breaking backwards compatibility.

I am pushing this PR to get the tests to run. They won't run on my branch on my fork. Please disregard until green.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.40%. Comparing base (5979c67) to head (9626777).
Report is 26 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5776      +/-   ##
==========================================
- Coverage   97.15%   94.40%   -2.75%     
==========================================
  Files         460      460              
  Lines        9559     9618      +59     
==========================================
- Hits         9287     9080     -207     
- Misses        272      538     +266     

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