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.