Fix shorthand_linter for most recent Sass gem

The internal structure changed between Sass versions. Previously, a
property such as:

    padding: 1px 1px 1px 1px;

Was stored as a `Sass::Script::String`, which the shorthand_linter was
explicitly sanity-checking. This changed to `Sass::Script::List`.
Fortunately, the `#to_s` returns the same thing, so the rest of the
linter works fine.

This drops the explicit class check for compatibility across versions,
letting duck-typing do its thing.

Change-Id: Ic9b1ce2d3a8ad48a284064fa24ac04fbd03612e4
Reviewed-on: https://gerrit.causes.com/19250
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>
Only scan files with `css` or `scss` extension

Now that recursive directory scanning is working, it could be helpful to
pass a directory to `scss-lint` and have it only scan files that look
like they need to be linted.

Change-Id: I8eb2d49516b1514670f608b1f6c66c2e78615048
Reviewed-on: https://gerrit.causes.com/19253
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>