Suggest a runnable gem pristine command

Suggests a gem pristine command that is ready to be run, when warning about missing extensions. For example the missing extensions warning for puma v2.9.1 would now suggest:

  `Try: gem pristine puma --version 2.9.1`

which succeeds, instead of the previous:

  `Try: gem pristine puma-2.9.1`

which would have resulted in `Failed to find gems ["puma-2.9.1"]`.