add reinstall option to avoid running gem pristine on all gemsets

I have dozens of projects with fairly large gemsets (140 gems are typical).

I also reinstall patched ruby versions rather frequently.

Running gem pristine on all of them after reinstalling the patched ruby version takes a looong time.

In 99% of the reinstalls the ruby ABI is not changed, making the gem pristine superfluous.

Could you add a flag to reinstall to suppress the gem pristine commands?