ruby plugin: support new download URL

[kyrofa]

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This PR resolves LP: #1815336 by supporting the new URL required to download Ruby 2.6 and above (note that it's backward compatible, the older versions are available at this URL as well).

[codecov-io]

Codecov Report

Merging #2466 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2466      +/-   ##
==========================================
+ Coverage   89.89%   89.89%   +<.01%     
==========================================
  Files         196      196              
  Lines       13041    13043       +2     
  Branches     1976     1976              
==========================================
+ Hits        11723    11725       +2     
  Misses        905      905              
  Partials      413      413

Impacted Files	Coverage Δ
snapcraft/plugins/ruby.py	98.86% <100%> (+0.02%)	⬆️

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[cmatsuoka]

Correctly extracts feature from version number and add it to URL.