update postcss to version 7 (#150)

Author: EECOLOR

package.json

@@ -46,7 +46,7 @@
   },
   "homepage": "https://github.com/css-modules/postcss-modules-extract-imports",
   "dependencies": {
-    "postcss": "^6.0.1"
+    "postcss": "^7.0.5"
   },
   "devDependencies": {
     "babel-cli": "^6.24.1",