Fix ES Module Output

[timjchin]

This PR makes two changes:

• The rollup referenced in npm run build was swapped to use the executable found in node modules
• The format for the rollup config was changed to use es module output, I'm not certain that es is a valid option for format (search for output.format), and the output from rollup didn't seem to contain our export?

[patrick-fs]

@timjchin Interesting observation about the output value. I actually used rollup's own lib example config as the basis for my own: https://github.com/rollup/rollup-starter-lib/blob/master/rollup.config.js

I've verified as well that the resulting build artifact (dist/index.js) does indeed export a default module:

Having said that, it's possible that the rollup example is stale. I'll update the config per the docs (and may even to a PR for rollup's example).

[timjchin]

Closing this in favor of https://github.com/fullstorydev/fullstory-browser-sdk/pull/11!