update storybook to 4.1.18

I noticed the storybook hosted at https://blockchain.github.io/components/ fails to load. A quick search points to a transitive dependency issue described here: storybookjs/storybook#6492

Updating storybook to the latest minor version resolves the issue, at least locally for me.

package.json

@@ -53,13 +53,13 @@
     "@babel/preset-react": "7.0.0",
     "@commitlint/cli": "7.3.2",
     "@commitlint/config-conventional": "7.3.1",
-    "@storybook/addon-actions": "4.1.6",
-    "@storybook/addon-backgrounds": "4.1.6",
-    "@storybook/addon-info": "4.1.6",
-    "@storybook/addon-links": "4.1.6",
-    "@storybook/addon-notes": "4.1.6",
-    "@storybook/cli": "4.1.6",
-    "@storybook/react": "4.1.6",
+    "@storybook/addon-actions": "4.1.18",
+    "@storybook/addon-backgrounds": "4.1.18",
+    "@storybook/addon-info": "4.1.18",
+    "@storybook/addon-links": "4.1.18",
+    "@storybook/addon-notes": "4.1.18",
+    "@storybook/cli": "4.1.18",
+    "@storybook/react": "4.1.18",
     "@storybook/storybook-deployer": "2.8.1",
     "@svgr/cli": "4.1.0",
     "@svgr/core": "4.1.0",