canvas and sharp library conflict

## Issue or Feature
* when i use canvas and sharp, Some problems blocked the process

objc[2275]: Class GNotificationCenterDelegate is implemented in both /***/node_modules/canvas/build/Release/libgio-2.0.0.dylib (0x10cfd0498) and /***/node_modules/sharp/vendor/lib/libgio-2.0.0.dylib (0x10d612498). One of the two will be used. Which one is undefined.

## Steps to Reproduce

```js
const Canvas = require('canvas');
const sharp = require('sharp');
var canvas = Canvas.createCanvas(200, 200);
var ctx = canvas.getContext('2d');
// etc.
```

## Your Environment
* node-canvas 2.3.1:
* sharp 0.21.3:
* typescript 3.3.3:
* node 10:
* Mac OS X 10.13.6: