fix(package): update postcss to version 7.0.13

Closes #7

Author: greenkeeper[bot]

package.json

@@ -25,7 +25,7 @@
     "express": "^4.16.2",
     "html-minifier": "^3.5.8",
     "mkdirp": "^0.5.1",
-    "postcss": "^7.0.0",
+    "postcss": "^7.0.13",
     "puppeteer": "^1.4.0",
     "uglify-js": "^3.3.7"
   },