ruby 2.6 support

[flavorjones]

Placeholder story to cut a release supporting Ruby 2.6.

Some prior tickets to look at to make sure we're doing everything right:

• Support Ruby 2.2 on Windows #1256
• Release Windows fat gem for Ruby 2.4 #1618
• Release Windows fat gem for Ruby 2.5 #1706
• Make nokogiri compatible to ruby 2.5 on Windows #1709

We'll need to:

• add 2.6 to the Windows gem
• update Concourse pipelines to promote 2.6 to main pipeline (see flavorjones/concourse-gem@5c1d1f2)

See also #1841 which discusses removing 2.2. To avoid having an overly-fat binary Windows gem, let's considering doing both in one fell swoop.

[larskanis]

I just prepared rake-compiler-dock for ruby-2.6 rake-compiler/rake-compiler-dock@560dbf2 and will ship rake-compiler-dock-0.6.4 (or 0.7.0 - not sure currently) with the final ruby-2.6.0 build on Christmas. Similar plan to release RubyInstaller-2.6.0 promptly. Subsequently I'll open a PR for the Windows build of Nokogiri. Would be great if you @flavorjones can make a release afterwards, so that the gap isn't too long. Is that OK?

[flavorjones]

@larskanis That sounds great, planning to cut v1.10.0 as soon as possible after ruby 2.6 final is out. Thank you!

[flavorjones]

Reopening