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.
@robwierzbowski it is manually pushed into the `args` array which `dargs()` generates ([here](https://github.com/sindresorhus/gulp-ruby-sass/pull/114/files#diff-168726dbe96b3ce427e7fedce31bb0bcR100)).

I believe this is necessary because dargs will prefix all options with `--`, and `update: 'paths'` no longer works as that generates `--update=paths` which Sass does not understand.