fix: package.json & yarn.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-HIGHLIGHTJS-1045326

Author: snyk-bot

package.json

@@ -6,7 +6,7 @@
     "react-scripts": "0.6.1"
   },
   "dependencies": {
-    "highlight.js": "^9.7.0",
+    "highlight.js": "^9.18.2",
     "react": "^16.5.0",
     "react-dom": "^15.3.2",
     "react-router": "^2.8.1",

yarn.lock

@@ -2384,9 +2384,10 @@ header-case@^1.0.0:
     no-case "^2.2.0"
     upper-case "^1.1.3"
 
-highlight.js@^9.7.0:
-  version "9.7.0"
-  resolved "https://registry.yarnpkg.com/highlight.js/-/highlight.js-9.7.0.tgz#e7a926bf3079c65b2ae50314878e456a009b4aac"
+highlight.js@^9.18.2:
+  version "9.18.5"
+  resolved "https://registry.yarnpkg.com/highlight.js/-/highlight.js-9.18.5.tgz#d18a359867f378c138d6819edfc2a8acd5f29825"
+  integrity sha512-a5bFyofd/BHCX52/8i8uJkjr9DYwXIPnM/plwI6W7ezItLGqzt7X2G2nXuYSfsIJdkwwj/g9DG1LkcGJI/dDoA==
 
 history@^2.1.2:
   version "2.1.2"