feat: update to webpack4+

[HOUCe]

In webpack 4, Tapable#apply and Tapable#plugin raise a deprecation notice.

• Tapable#apply(plugin) must be replaced by a call to plugin.apply
• Tapable#plugin(hook, fn) must be replaced by a call to hook.tap (or hook.tapAsync)

Note: This is breaking change because it drops webpack < 4 compatibility.

For more info:

• Highly suggest to refractor with Target design which uses webpack native plugins such as JsonpTemplatePlugin. This leads to incompatibility with lower webpack version.
• Update wxapp-boilerplate needed.
• webpack4+ does not support node <= 4, need to delete Travis CI test version.