Travis Build Issue (RubyGems)

See: https://github.com/bitcoinops/bitcoinops.github.io/pull/1435

RubyGems dropped support for 2.6.X which we use.

https://app.travis-ci.com/github/bitcoinops/bitcoinops.github.io/builds/267901948#L250

https://github.com/rubygems/rubygems/pull/7116