Suggest a runnable gem pristine command

When warning about missing extensions, suggest a `gem pristine` command that is ready to be run. For example, the missing extensions warning for puma v2.9.1 now suggests `Try: gem pristine puma --version 2.9.1`, which succeeds. The previous suggestion was `Try: gem pristine puma-2.9.1`, which results in `Failed to find gems ["puma-2.9.1"]`.