Teach scss-lint `property: 10px 10px` can be shorter

The linter now complains about shorthand styles that follow the pattern
`property: 10px 10px`.

Related Story:
  http://www.pivotaltracker.com/story/show/43148129 (delivered)
  ("Teach scss-lint about missed shorthand case")

Change-Id: If8034ad68b0fe81afca64ba4228fda49e70f4bc5
Reviewed-on: https://gerrit.causes.com/17749
Reviewed-by: Lann Martin <lann@causes.com>
Tested-by: Joe Lencioni <joe.lencioni@causes.com>