Downgrades puppeteer to ~1.11.0

This downgrades puppeteer to version `~1.11.0`.
This will download the stable version of chromium
(72.0.3582.0). The current puppeteer version
(`^1.12.2`) downloads the beta version of
chromium (73.0.3679.0).

package.json

@@ -79,7 +79,7 @@
     "pixelmatch": "^4.0.2",
     "pngjs": "^3.3.3",
     "proj4": "2.5.0",
-    "puppeteer": "^1.12.2",
+    "puppeteer": "~1.11.0",
     "serve-static": "^1.13.2",
     "shx": "^0.3.2",
     "sinon": "^7.2.3",