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.

lib/mutant/killer/rspec.rb

@@ -20,12 +20,7 @@ module Mutant
         # TODO: replace with real streams from configuration
         require 'stringio'
         null = StringIO.new
-        argv = command_line_arguments
-        begin
-          !::RSpec::Core::Runner.run(argv, null, null).zero?
-        rescue StandardError
-          true
-        end
+        !::RSpec::Core::Runner.run(command_line_arguments, null, null).zero?
       end
 
       # Return command line arguments