Update webpack packages

[jnm]

See webpack/webpack#8082 (comment).

Fixes this error during the build process:

> webpack --config webpack/prod.config.js --progress --colors

/srv/node_modules/webpack-cli/bin/config-yargs.js:89
                describe: optionsSchema.definitions.output.properties.path.description,
                                                           ^

TypeError: Cannot read property 'properties' of undefined
    at module.exports (/srv/node_modules/webpack-cli/bin/config-yargs.js:89:48)
    at /srv/node_modules/webpack-cli/bin/webpack.js:60:27
    at Object.<anonymous> (/srv/node_modules/webpack-cli/bin/webpack.js:515:3)
    at Module._compile (module.js:653:30)
    at Object.Module._extensions..js (module.js:664:10)
    at Module.load (module.js:566:32)
    at tryModuleLoad (module.js:506:12)
    at Function.Module._load (module.js:498:3)
    at Module.require (module.js:597:17)
    at require (internal/module.js:11:18)