Ignore dynamic values in PlaceholderInExtend linter

The `PlaceholderInExtend` linter would incorrectly report a lint for
selectors whose prefix is dynamically generated with interpolation.

Since it cannot statically determine that the prefix is not a
placeholder, it incorrectly assumed that any string not beginning with
'%' could not possibly be a placeholder, which unfortunately included
interpolated strings (which start with '#{')

The solution is to check the first element of the `ExtendNode#selector`
array to see if is is a `Sass::Script::Tree::Node` (and thus dynamic
content).

Change-Id: Id9c1a553e1ab242472a871682371e436fb403ea0
Reviewed-on: http://gerrit.causes.com/39134
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>