Fix error when generating an external source map.

Fixes #7

Author: BenLubar

lib/clean-css-processor.js

@@ -17,6 +17,7 @@ module.exports = function() {
             if (sourceMap) {
                 options.sourceMap = sourceMap.getExternalSourceMap();
                 if (options.sourceMap) {
+                    options.sourceMap = options.sourceMap.toString();
                     var sourceMapObj = JSON.parse(options.sourceMap);
                     if (sourceMapObj.sourcesContent) {
                         sourcesContent = sourceMapObj.sourcesContent;