Add Ruby 2.5.9, 2.6.7, 2.7.3 and 3.0.1

Automatically pick the latest version of 2.6 for testing

* 2.6.7 is not yet available on Windows

Add JRuby 9.2.17.0

• #165

JRuby 9.3+ ships with Bundler 2

v1.66.1

Document the logic for whether to install Bundler

Add JRuby 9.2.16.0

v1.66.0

Add JRuby 9.2.16.0

Add JRuby 9.2.15.0

v1.65.0

Add JRuby 9.2.15.0

Use Bundler 1 on Ruby 2.3.0 and 2.3.1

• Fixes #148

Use Bundler 2 on Ruby 2.3 by default

• Now that rubygems/rubygems#3570 is fixed
• See #146

Add TruffleRuby 21.0.0

v1.63.0

TruffleRuby 21.0.0 works on macOS 11.0

Fix error message when trying to install TruffleRuby on Windows

v1.62.1

Fix error message when trying to install truffleruby on windows

Automatically install libxml2-dev libxslt-dev on TruffleRuby if needed with bundler-cache: true

• This is a special case, because TruffleRuby needs an extra system dependency that other Rubies do not, and GitHub runners do not have these packages installed by default, while TravisCI has them.