Fix disable_flash_sweep spec helper

Broken in previous commit 26ade602:

RSpec::Mocks.allow_message is a lower-level API method, allowing us
to define a stub outside of an ExampleGroup context.

Signed-off-by: Alex Coles <alex@alexbcoles.com>

spec/support/flash.rb

@@ -28,6 +28,5 @@
 #++
 
 def disable_flash_sweep
-  controller.instance_eval { allow(flash).to receive(:sweep) }
+  controller.instance_eval { RSpec::Mocks.allow_message(flash, :sweep) }
 end
-