Fix TrailingSemicolon linter for `@include`s

This is a follow-up to a couple of commits
(mainly b4e70aac and f955035ee, but also 58689b1d18) aiming to make the
TrailingSemicolon linter work for `@include`s and `@extend`s.

@blomeyer recently pointed out in #205 that this linter still has
problems with `@include`s. He gave the following repro case:

  $ cat test.scss
  @include foo(bar) {
    @if $var {
      color: #f00;
    }
  }

  @include foo(bar) {
    background: #000;
    @if $var {
      color: #f00;
    }
  }
  $  scss-lint test.scss
  test.scss:1 [W] TrailingSemicolon: Declaration should not have a space
  before the terminating semicolon
  $  scss-lint -version
  scss-lint 0.26.1

I took another stab at the code path that deals with mixins, and made it
so that mixins that have children (any kind) will ignore the current
mixin node and continue checking children. I kept the old
children-checking behavior for property nodes.

Now that the strategy has changed slightly, it might not make sense to
have all the test cases for `@include`s. Perhaps we could reduce them to
two paths: one where `@include` has children, and one where `@include`
does not have children. I decided to keep all different versions around
though, and add another one (similar to what @blomeyer provided in the
issue).

Change-Id: Icb18acb2ddaf555ba7028f2048fd5c106fe66be5
Reviewed-on: http://gerrit.causes.com/41048
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>