Moves Ruby version support window to ['2.7', '3.0', '3.1']

[mattxwang]

Ruby 2.4, 2.5, and 2.6 are all EOL! And, since Donald was last involved in this gem, Ruby 3 and 3.1 have been stabilized.

This PR:

• updates our CI to require checks to pass for ['2.7', '3.0', '3.1'] (note that ruby-head is still allowed to fail)
• bumps the gemspec's required ruby version
• deletes the old Travis config

[codecov]

Codecov Report

Merging #69 (35b38f5) into main (f6f6659) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
- Coverage   99.26%   99.25%   -0.01%     
==========================================
  Files           6        6              
  Lines         407      403       -4     
==========================================
- Hits          404      400       -4     
  Misses          3        3              

Impacted Files	Coverage Δ
lib/sorbet-coerce/converter.rb	97.36% <0.00%> (-0.09%)	⬇️

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[mattxwang]

Going to be honest, don't understand why the codecov check is failing - the impacted file shows nothing!