Update ruby supported version to > 2.2

[vfonic]

Description

Since you dropped support for 2.2, there was one place left in the README where it wasn't addressed.

• Relevant Issues : (none)
• Relevant PRs : (none)
• Type of change :
  • Addition or Improvement of documentation

[codecov-io]

Codecov Report

Merging #289 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #289   +/-   ##
=======================================
  Coverage   85.71%   85.71%           
=======================================
  Files           7        7           
  Lines         434      434           
=======================================
  Hits          372      372           
  Misses         62       62

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[avdv]

Thank you! Which reminds me, Ruby 2.3 is already EOL too...