Publish to RubyGems on tagged version

[JakeQuilty]

What does this PR do?

Changes the publish to RubyGems process to publish on push to master with a tagged(v*) release. Ex) v5.3.2

More notes in the issue.

What ticket does this PR close?

Connected to https://github.com/cyberark/conjur-api-ruby/issues/173

Checklists

Change log

• [ ] The CHANGELOG has been updated, or
• [ x ] This PR does not include user-facing changes and doesn't require a CHANGELOG update

Test coverage

• [ ] This PR includes new unit and integration tests to go with the code changes, or
• [ x ] The changes in this PR do not require tests

Documentation

• [ x ] Docs (e.g. READMEs) were updated in this PR, and/or there is a follow-on issue to update docs, or
• [ ] This PR does not require updating any documentation

[JakeQuilty]

@sgnn7 I put a lot of that info in the issue(https://github.com/cyberark/conjur-api-ruby/issues/173#issuecomment-675156079). I'll copy it over here too

[codeclimate[bot]]

Code Climate has analyzed commit ee45639d and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 92.0% (0.0% change).

View more on Code Climate.