CI: Build gems with strictness and verbosity & Add upper version limits to gemspec

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (670f0a9) 45.16% compared to head (26c0364) 45.16%.

:exclamation: Current head 26c0364 differs from pull request most recent head cb7eab4. Consider uploading reports for the commit cb7eab4 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #135 +/- ## ======================================= Coverage 45.16% 45.16% ======================================= Files 4 4 Lines 186 186 ======================================= Hits 84 84 Misses 102 102 ```

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[bastelfreak]

This will require us to drop Ruby 2.5