Remove rescue from rspec killer

* Based on recent discussions in #68 we think rescuing all
  exceptions like this was probably a mistake.

  If this continues to be a problem we will need to narrow things
  down to a reproducible issue we can report to the RSpec team.

  One possible work-around might be to catch exceptions that occur
  within the specs themselves, and to bubble up everything else.
  However, this does bring up the question of why the specs failed;
  if it was due to a syntax error caused by mutant and unparser,
  then that's probably a bug we need to fix. It could be for some
  other reason, which we would have to isolate and handle
  explicitly, but I think we are all in agreement that this logic is
  too broad.