CI: Remove Bundler 1.x workaround

[olleolleolle]

The "github" source now has that form, already.

This PR only serves to simplify that Gemfile.

[codecov]

Codecov Report

Merging #296 (38b6d60) into master (45e8769) will not change coverage.
The diff coverage is n/a.

❗ Current head 38b6d60 differs from pull request most recent head 243139f. Consider uploading reports for the commit 243139f to get more accurate results

@@            Coverage Diff            @@
##            master      #296   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           50        50           
  Lines         1868      1868           
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  Hits          1868      1868           

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[ddnexus]

Thank you!

This PR was failing in travis.ci. Then, since it's an intermittent problem happening since a few days, I just re-built it and it worked this time.

I think it's related to the warning of codecov circular require, which is happening because simplecov requires it on its own with docile when on travis and on ruby 3.0, and there is a require in the .simplecov file too. I will fix that next ;).

Thanks