Remove support for Webpack <= v3 in the next major

[satazor]

• Update package.json peer & dev dependency
• Remove custom Travis script
• Remove v2 and v3 from README
• Update all webpack config files in test fixtures to have the mandatory mode property set to development
  • Additionally, remove the supportsMode from util/createCompiler
• Simplify lib/devMiddleware.js; we may also remove notifyDone