Add support for truffleruby

[donv]

https://github.com/oracle/truffleruby

[ivoanjo]

+1 I'm seeing

RubyDep: WARNING: Your Ruby may not be supported.
RubyDep: WARNING: Your Ruby is: 2.6.2 'truffleruby' (unrecognized). If you need this version supported, please open an issue at http://github.com/e2/ruby_dep

in my logs :)

[eregon]

There is #37 but the maintainer @e2 seems unresponsive (and this gem seems dead).
I think I'll just automatically suppress those warnings on TruffleRuby.

[eregon]

truffleruby/truffleruby@b88bae6 should silence warnings from ruby_dep, so I think we can close this issue.