Possible fix for error described in PR#1523

In reference to commit 5a8aec1 and [related disucssion](https://github.com/am11/WebEssentials2013/commit/5a8aec1015cf5eeb58481d39539d348b35c9a15d#commitcomment-7879628) I did a sample test case (outside of VS environment) and the error I received was `Unsupported previous source map format`, I did some tracing and noticed the following:

1. postcss options, expect previous source map to be of type `string`.
2. `result.map` returned by both rtlcss and autoprefixer are of type `object`.

Moreover, in LESS compilation flow, rtlcss already recieved the prefixed css, and need not to call autoprefixer service again.





Should we do the same with SCSS: https://github.com/MohammadYounes/WebEssentials2013/blob/22d399e1b08caf6f8c1ba3a2b8bf6bf9b2b0f531/EditorExtensions/Resources/server/services/srv-scss.js#L45-L50

If that is the case, then we can change the sequence of parameters and get rid of undefined from caller and push those parameters to the end in function signature.