Remove transactions and temporarily disable group

It does not seem likely that we will be able to stabilize
transactions in a general way any time in the near future.
I would like to restore a somewhat limited version of
the group() feature that does not rely on transactions,
but until we do, disabling the feature will make the
problem obvious right away to users rather than much
later via some hard-to-debug symptom.
Drop :optimize_objects feature because it is no longer used
Drop use of GC in soft mask
Use a version of PDF::Core without GC
Merge pull request #676 from prawnpdf/remove_transactions

Remove transactions and temporarily disable group