Upgrade minimum Rubocop version to 0.13.0

This ensures that the fix reported in:

  https://github.com/bbatsov/rubocop/issues/483

is now incorporated into Rubocop, preventing `haml-lint` from crashing
in cases where a variable was referenced in an `if` statement before it
was initialized (see link for details).

Change-Id: I6c8b3d88535f679933c40ac8deed3cf68d1f08b3
Reviewed-on: https://gerrit.causes.com/28634
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>