Prevent buffet from clearing shared examples

Commit 8e8fb2b of rspec-core introduced a change that would clear all
shared examples when `Rspec.world.reset` was called. This causes any
global shared examples (for example, loaded in `spec_helper` for all
example groups) to only be present on the first run of buffet and
missing on all subsequent runs.

This commit addresses this by no longer calling `world.reset`, but
instead reaching directly into `world.example_groups` to clear only
the loaded example groups.

Change-Id: I729c0ab1d26f0fda091f656459cb2574664ae608
Reviewed-on: https://gerrit.causes.com/19817
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Tony Wooster <tony.wooster@causes.com>