Gives warning at npm install regarding deprecated gulp-util

npm install for gulp-spawn-mocha-nyc gives the following warning:
```
npm WARN deprecated gulp-util@3.0.8: gulp-util is deprecated - replace it, following the guidelines at https://medium.com/gulpjs/gulp-util-ca3b1f9f9ac5
```
It looks like this would be simple to fix by using:
https://www.npmjs.com/package/plugin-error
...rather than the one from gulp-util.
