fix: the mockjs library make development environment crashed

[likui628]

closes #208

由于mock.js的package.json里没有node-canvas依赖倒置的
https://github.com/nuysoft/Mock/blob/c4d7cba01900b5c5bb8e3d474c8f5d07810ab72e/src/mock/random/image.js#L242-L251

先将图片用pravatar上的图片替换。

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