We fixed some issues recently where Bundler would try to activate a
pysch spec with missing extensions and crash. However, as a side effect,
we started printing warnings about missing extensions in situations
where we did not warn before.

It may be interesting to warn on these new situations too, but in order
to minimize changes for now, I'm reverting to printing warnings in the
same situations as before.

https://github.com/rubygems/rubygems/commit/51ebff6982