chore(package): update postcss to version 7.0.3

Closes #82

Author: greenkeeper[bot]

package.json

@@ -63,7 +63,7 @@
     "flow-bin": "^0.61.0",
     "mocha": "^5.0.0",
     "nyc": "^11.0.1",
-    "postcss": "^6.0.7",
+    "postcss": "^7.0.3",
     "postcss-modules": "^1.0.0",
     "sinon": "^4.0.1",
     "sinon-chai": "^2.8.0",