CI: Drop unused setting, update build matrix, resolve version-specific depdendencies

[olleolleolle]

This PR updates the CI matrix:

• Updates Ruby patch versions in matrix to 2.4.6, 2.5.5
• Adds Ruby 2.6.2 for Rails 6
• Removes sudo: false, an old Travis setting. See https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

---

Build failures

Issues dealt with:

• Update ZenTest to a newer version compatible with RubyGems 3.0.0+
• Use Ruby 2.5+ with Rails 6
• Rails 5.0.7.2, Ruby 2.3: Gem::LoadError: can't activate sqlite3 (~> 1.3.6), already activated sqlite3-1.4.0. Make sure all dependencies are added to Gemfile.
• Rails 4.2.11.1, Ruby 2.4: Same.
• Rails 3: Same, for sqlite ~> 1.3.5

The build failures about sqlite3 are explained in this StackOverflow thread.

[JonRowe]

Don't suppose you care to try to tackle the build failures? I know they're unrelated!

[olleolleolle]

@JonRowe I'm doing it, burning cycles like mad.

[olleolleolle]

💚 @JonRowe Could you take a look and see how you find the change?

Question: Should we update to newer patch versions of Ruby in this change? Answer: Since you are on the other side of the globe, I added 2.6.2, 2.5.5, 2.4.6 where applicable.

[JonRowe]

thanks 💚

[olleolleolle]

Hooray!