fix(package): update styled-system to version 4.0.7

Closes #114

Author: greenkeeper[bot]

package.json

@@ -72,7 +72,7 @@
     "react-dom": "^16.3.1",
     "react-ga": "^2.4.1",
     "styled-components": "^3.2.5",
-    "styled-system": "^3.0.0"
+    "styled-system": "^4.0.7"
   },
   "devDependencies": {
     "@storybook/addon-actions": "^3.4.1",