Don't report usage lint for hyphenated keyword args

There was a bug where any time a user used keyword arguments, e.g.

    @include line-clamp($line-count: 3, $line-height: 1.5em);

...lints would be incorrectly reported for each hyphenated keyword
argument.

This was due to the Sass parser treating variables with underscores as
canonical. Since the `Sass::Script::Variable` `underscored_name`
attribute was used to get the name of the variable under the hood, even
if a variable was written as `$some-var`, the linter would see it as
`$some_var` and report a lint.

The easy solution for this was to monkey-patch
`Sass::Script::Variable#underscored_name` to return the original name.
A more proper fix would have been to contribute upstream to the `sass`
gem, but that potentially requires us to maintain a separate fork in the
event that patch isn't accepted.

Fixes #27

Change-Id: I27b539c2f68035284d22bacd2638e7487cab5254
Reviewed-on: https://gerrit.causes.com/27819
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Lann Martin <lann@causes.com>