Update webpack and change Babel settings

[trueadm]

Release notes: Prepack now ships Node 6.1+ compatible source

This PR does four things:

• updates webpack to v4
• changes our Babel presets so we now transpile far less code, in fact we now target node 6.1+ min (as we already specified in package.json). The bundle created for the REPL still targets ES5.
• updated eslint
• updated circleci node version to 9.7.1 (yarn says it doesn't support it, but there appears to be no issues)

This should improve runtime performance and make debugging easier. In fact, there's no real need for sourcemaps now, as the code looks almost identical minus module references.

Performance comparisons:

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