json-schema.gemspec: Raise required ruby version to 2.5

[bastelfreak]

That's the version we currently test on.

[codecov]

Codecov Report

Merging #466 (1212a6a) into master (b0983fa) will not change coverage.
The diff coverage is n/a.

❗ Current head 1212a6a differs from pull request most recent head 7937677. Consider uploading reports for the commit 7937677 to get more accurate results

@@           Coverage Diff           @@
##           master     #466   +/-   ##
=======================================
  Coverage   89.83%   89.83%           
=======================================
  Files          74       74           
  Lines        1555     1555           
=======================================
  Hits         1397     1397           
  Misses        158      158           

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