Drop bundler dev dependency

[bastelfreak]

We don't have it in the majority of our gems. To keep the setup identical, we can remove it in json-schema as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.52%. Comparing base (a75665f) to head (48eecc2).
Report is 34 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #524      +/-   ##
==========================================
- Coverage   90.58%   90.52%   -0.07%     
==========================================
  Files          76       76              
  Lines        1572     1562      -10     
==========================================
- Hits         1424     1414      -10     
  Misses        148      148              

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