Update prefixSelector for latest selector parser

Author: adamwathan

src/util/prefixSelector.js

@@ -1,11 +1,17 @@
 import parser from 'postcss-selector-parser'
+import get from 'lodash/get'
 
 export default function(prefix, selector) {
   const getPrefix = typeof prefix === 'function' ? prefix : () => prefix
 
   return parser(selectors => {
     selectors.walkClasses(classSelector => {
-      classSelector.value = `${getPrefix('.' + classSelector.value)}${classSelector.value}`
+      const baseClass = get(classSelector, 'raws.value', classSelector.value)
+
+      classSelector.setPropertyAndEscape(
+        'value',
+        `${getPrefix('.' + baseClass)}${baseClass}`
+      )
     })
   }).processSync(selector)
 }