chore: make sure Puppeteer bundling works (#3239)

This patch:
- adds "browser" field to the package.json with default
  bundling options.
- introduces "bundle" and "unit-bundle" commands to
  create bundle and test bundle
- starts running bundle tests on Travis Node 8 bots

Fixes #2374.

.gitignore

@@ -11,3 +11,4 @@ package-lock.json
 yarn.lock
 /node6
 /lib/protocol.d.ts
+/utils/browser/puppeteer-web.js