Update percy requirement from ^0.14.0 to ^0.15.0

Updates the requirements on [percy](https://github.com/percy/percy-agent) to permit the latest version.
- [Release notes](https://github.com/percy/percy-agent/releases)
- [Changelog](https://github.com/percy/percy-agent/blob/master/CHANGELOG.md)
- [Commits](percy/percy-agent@v0.14.0...v0.15.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

package.json

@@ -46,7 +46,7 @@
     "eslint-plugin-standard": "^4.0.0",
     "npm-run-all": "^4.1.5",
     "parcel-plugin-bundle-visualiser": "^1.2.0",
-    "percy": "^0.14.0",
+    "percy": "^0.15.0",
     "postcss-cli": "^6.1.3",
     "postcss-fontpath": "^1.0.0",
     "posthtml-expressions": "^1.1.1",