Attempt to make npm audit happy

I ran these commands to generate this diff:

  - `npm update`
  - `npm audit fix`

After these changes, I still see some noise:

```
$ npm audit
...
found 8 low severity vulnerabilities in 793 scanned packages
  8 vulnerabilities require manual review. See the full report for details.
```

These stragglers all are indirectly related to these top level
dependencies that I think may be unmaintained now:

  - `gulp-svg2png`
  - `gulp-jimp`
  - `gulp-iconfont`