Remove `proc_to_lambda`.

There's a very odd ruby bug that makes this unsafe to use, as its
usage mutates the behavior of the block:

https://bugs.ruby-lang.org/issues/9967

Unfortunately, this un-fixes rspec/rspec-mocks#714, but we've got to do it.
Merge pull request #85 from rspec/remove-proc-to-lambda

Remove `proc_to_lambda`.