Fix UnnecessaryParentReference with multiple parent refs

The `UnnecessaryParentReference` linter would incorrectly report lints
for cases like the following:

    p {
      & + & { ... }
    }

In these situations, the first ampersand is required, since if you wrote
something like this:

    p {
      + & { ... }
    }

...the Sass compiler would produce the following invalid CSS:

    + p { ... }

Fix the problem by detecting if there are multiple ampersand references
in the sequence and ignoring it if that is the case.

Change-Id: Ica6042785883e3ed1c76d24dd8a25da5899bdfda
Reviewed-on: http://gerrit.causes.com/41625
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>