Update dargs dependency, fix tests to work with Sass 3.4 and dargs 2.x

grunt-contrib-sass has already updated to dargs 2.0.0, so I believe this should be safe enough.

This update allows one to specify the Sass 3.4's `sourcemap` option.

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**Update:**

- Some tests are failing in Windows because Vinyl files contain OS-dependent dir separators and the unit tests have forward slashes hardcoded in. I've replaced those with `path.join`;

- For the unit tests to run correctly with Sass 3.4.1 (which has sourcemaps enabled by default), we have to manually disable sourcemap generation when sourcemaps shouldn't be generated (`sourcemap: 'none'`). I'm removing `sourcemap: true` as well as that displays a deprecation warning;

- There is a conflict with dargs 2.x here that is generating an `--update=path:path` option which is not accepted by Sass. I've patched this by turning update into a boolean option and adding the `srcPath:destPath` manually to the args list.
> or is generating args without leading -- something that dargs should handle as well?

No