download Chromium only when tests are run

package.json

@@ -36,7 +36,7 @@
     "gh-pages": "^1.0.0",
     "jsdoc": "^3.5.5",
     "json": "^9.0.4",
-    "puppeteer": "^2.0.0",
+    "puppeteer-core": "^2.0.0",
     "scratch-vm": "0.2.0-prerelease.20191227164934",
     "tap": "^11.0.0",
     "travis-after-all": "^1.4.4",