[Bug Report] Cannot build Element with Node 12

### Element UI version
2.13.0

### OS/Browsers version
Ubuntu 18.04

### Vue version
2.6.11

### Reproduction Link
https://github.com/ElemeFE/element/

### Steps to reproduce
While investigating #11353, I faced an issue with building Element using current LTS Node v12. Compilation of native addons fails with
```
‘class ...’ has no member named ‘IsNearDeath’
```

It seems that at least the locked versions of `node-sass` (`4.11.0`) and `nan` (`2.12.1`) do not support Node 12.
`node-sass` introduces this support from [version 4.12.0](https://github.com/sass/node-sass/releases) and `nan` from [version 2.14.0](https://github.com/nodejs/nan/tree/v2.14.0).

I did not manage to upgrade only these dependencies, so suggesting a PR with the full `yarn upgrade`. With these changes Element is built successfully on Node 12.

### What is Expected?
Element can be built with Node 12

### What is actually happening?
Native addons compilation failures