Use ruby greater than 2.6

[vrr-21]

Issue #, if available:

Description of changes:
Upgrading the acceptable ruby version to 2.6.0 or greater as Ruby versions below 2.5 are now end-of-life.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[coveralls]

Coverage remained the same at 47.849% when pulling dcc4c43 on ruby-upgrade into 93c0149 on master.

[vrr-21]

Removing Ruby backwards compatibility might be a breaking change for many customers. Hence we'd like to exercise caution around that.

[ajanata]

Adding the requirement for 2.3.8 in 2f74f7d actively broke us; we don't have that version installed and for whatever reason it refuses to install on our boxes and also prevents us from upgrading it. Can you consider removing .ruby-version altogether?

[vrr-21]

This PR is closed, and will not be monitored.

Can you open an issue with this request? We'd be happy to take a look at it and explore any possibilities.

[ajanata]

And I'm saying it was closed for an invalid reason. There was no forced Ruby version until 2 months ago, at which time it was forced to be version that's been out of support for over 3 years. Either revert that change or re-open and merge this. You're worried about breaking existing customers with this change, but you already did that 2 months ago.

[philstrong]

We haven't heard from anyone else that this is an issue. What version of Ruby are you trying to use? What OS platform?