fix: added postcss in devDependencies for testing

Author: Mathieu Lavoie

package.json

@@ -52,6 +52,7 @@
     "gulp-sourcemaps": "^2.6.5",
     "mocha": "^5.2.0",
     "nyc": "^12.0.2",
+    "postcss": "^8.0.0",
     "proxyquire": "^2.1.0",
     "sinon": "^6.3.5",
     "vinyl": "^2.2.0"