Refactor mkmf with safe navigation operator and split with block

[Maumagnaguagno]

With safe navigation operator (Ruby 2.3) and split with block (Ruby 2.6) it is possible to avoid the intermediate variable opt and array created by split, resulting in a more compact and clear code for the mkmf method.

[Maumagnaguagno]

I just noticed that the un.gemspec contains Ruby version >= 2.3.0, while String#delete_prefix! (Ruby 2.5) is used in the setup method since d96a5e5.

Should I open a new PR to update to Ruby 2.5 to fix this first, or could the current PR with an updated Gemspec to Ruby 2.6 be enough?
EDIT: I just bumped the required Ruby version to 2.6.

[Maumagnaguagno]

irb just dropped Ruby 2.5 support, ruby/irb#383, while un still uses 2.3 in the un.gemspec, despite using new Ruby 2.5 String methods.
This PR bumps the requirements to Ruby 2.6, which already reached EOL.