Remove spread operator from umd bundle

[dawsbot]

We're getting prod errors in the Everipedia.org site for browsers which do not support the spread operator (...).

After running this repo locally, it appears that dfuse-client.umd.js contains this syntax, which should not land in user's browsers untranspiled.

None of the other dist outputs contain this, so I'm going to borrow their build syntax and PR this 🙌

[maoueh]

The problem seems to be that https://www.npmjs.com/package/debug which is the only dependency built into the UMD build is not compatible with ES5 anymore (they now refuse to do it)

So the only option is using Babel that would transpile the debug library correctly, like you did in your PR.

I'll review the PR more thoroughly this week and take a decision. Thanks for opening and fixing the issue :)