Update to Webpack@3

[mattlubner]

Changes

1. Webpack, plugins, loaders, and configuration files updated to work with Webpack@3

Reverted Changes

The following changes are still needed, but I will submit them in a separate PR once I've isolated the issue with the "Should correctly give number of Posts" test intermittently failing.

1. Removal of node --harmony-proxies flag from npm test script, for using Mern with a modern version of Node.js
2. Minimum engine requirement bumped to >=6.5 in package.json
3. Node versions tested in .travis.yml changed to LTS versions + latest (6.5, 8, 9)

Note that the removal of the --harmony-proxies flag is necessary as of Node v6.5, as the V8 engine no longer supports it.

[mattlubner]

There seems to be a race condition in server/models/__tests__/post.spec.js that is intermittently causing the "Should correctly give number of Posts" test to fail.

[mattlubner]

Hi @somus, are you the right person to ask about getting a PR reviewed & merged? 😀 I wasn't able to find any information on contributing to Mern (aside from the Code of Conduct).