chore(package): update @storybook/addon-knobs to version 5.1.8

Closes #90

Author: greenkeeper[bot]

package.json

@@ -46,7 +46,7 @@
     "@babel/preset-env": "^7.4.5",
     "@babel/preset-react": "7.0.0",
     "@storybook/addon-actions": "5.1.8",
-    "@storybook/addon-knobs": "5.1.1",
+    "@storybook/addon-knobs": "5.1.8",
     "@storybook/addon-links": "5.1.1",
     "@storybook/addon-storyshots": "5.1.1",
     "@storybook/addons": "5.1.1",