chore(package): update ember-cli-postcss to version 4.2.0

Closes #82

Author: greenkeeper[bot]

package.json

@@ -84,7 +84,7 @@
     "ember-cli-htmlbars": "^1.0.10",
     "ember-cli-htmlbars-inline-precompile": "^0.3.3",
     "ember-cli-inject-live-reload": "^1.4.1",
-    "ember-cli-postcss": "^3.1.1",
+    "ember-cli-postcss": "^4.2.0",
     "ember-cli-qunit": "^3.0.1",
     "ember-cli-release": "^0.2.9",
     "ember-cli-sri": "^2.1.0",