Fix TrailingSemicolon handling of comma-separated imports

The linter would incorrectly report lints for comma-separated imports
like the following:

    @import 'functions/strip-units',
            'functions/pc';

This was caused by the fact that the `sass` parser still creates
a parse node for each `@import`. Fix #294 by ignoring imports whose
source range ends with a comma.

Change-Id: I7c69a3b777fdb31ab092bb35aa6f9c2bc4c005c0
Reviewed-on: http://gerrit.causes.com/44731
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>