Update linear gradient syntax to avoid deprecation
Update linear gradient syntax to avoid deprecation

This updated syntax avoids the following deprecation warning in Chrome Canary v60.0.3090.0:
```[Deprecation] -webkit-gradient is deprecated. Please use linear-gradient or radial-gradient instead.```

This syntax is also supported in Firefox (and [all other modern browsers](https://caniuse.com/#feat=css-gradients)), which could help transitioning the Firefox version, which is in the works.

poacher2k/vimium,smblott-github/vimium,decaffeinate-examples/vimium,decaffeinate-examples/vimium,mrmr1993/vimium,poacher2k/vimium,mrmr1993/vimium,philc/vimium,mrmr1993/vimium,decaffeinate-examples/vimium,philc/vimium,gdh1995/vimium,gdh1995/vimium,philc/vimium,poacher2k/vimium,smblott-github/vimium,gdh1995/vimium,smblott-github/vimium