Bump pixelmatch from 5.0.2 to 5.1.0

Bumps [pixelmatch](https://github.com/mapbox/pixelmatch) from 5.0.2 to 5.1.0.
- [Release notes](https://github.com/mapbox/pixelmatch/releases)
- [Commits](https://github.com/mapbox/pixelmatch/compare/v5.0.2...v5.1.0)

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

package.json

@@ -82,7 +82,7 @@
     "marked": "0.7.0",
     "mocha": "6.2.2",
     "ol-mapbox-style": "^5.0.2",
-    "pixelmatch": "^5.0.0",
+    "pixelmatch": "^5.1.0",
     "pngjs": "^3.4.0",
     "proj4": "2.5.0",
     "puppeteer": "~1.20.0",