CI: Add truffleruby to the build matrix

[gogainda]

No description provided.

[gogainda]

@olleolleolle tests are passing but in the end rubocop fails with this exception:

Running RuboCop...

Error: RuboCop found unsupported Ruby version 2.3 in `TargetRubyVersion` parameter (in .rubocop.yml). 2.3-compatible analysis was dropped after version 0.81.

Supported versions: 2.4, 2.5, 2.6, 2.7, 2.8

RuboCop failed!

The command "bundle exec rake" exited with 1.
cache.2

store build cache

which is weird because truffleruby is 2.6 like

[olleolleolle]

@gogainda The issue is with the configuration file, which was created with an earlier version of RuboCop.

[gogainda]

Should I fix it or we can merge it?

[olleolleolle]

@gogainda Thanks for your contribution! I'll rebase this onto master which now builds green.