Bump prop-types from 15.6.1 to 15.6.2

Bumps [prop-types](https://github.com/facebook/prop-types) from 15.6.1 to 15.6.2.
- [Release notes](https://github.com/facebook/prop-types/releases)
- [Changelog](https://github.com/facebook/prop-types/blob/master/CHANGELOG.md)
- [Commits](facebook/prop-types@v15.6.1...v15.6.2)

Signed-off-by: dependabot[bot] <support@dependabot.com>

package.json

@@ -60,7 +60,7 @@
     "optipng-bin": "^4.0.0",
     "postcss": "^6.0.22",
     "postcss-cli": "^4.1.1",
-    "prop-types": "^15.6.1",
+    "prop-types": "^15.6.2",
     "react": "16.0.0",
     "react-dom": "16.0.0",
     "react-ga": "2.4.1",