Fix Indentation linter handling of @at-root

The `Indentation` linter would incorrectly report lines nested within
`@at-root` directives as having incorrect indentation.

This was due to us not including the `AtRootNode` in the list of visit
methods we defined in the `Indentation` linter.

Change-Id: Iaa2c4d4de07fd55d01f6ae5f4a1e3427de958417
Reviewed-on: http://gerrit.causes.com/37652
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>