Update strip-bom / Remove gulp-util from devDependencies

* I updated [strip-bom](https://github.com/sindresorhus/strip-bom) to v1.0.0.
* I removed [gulp-util](https://github.com/gulpjs/gulp-util) from `devDependencies`. [It's already included in `dependencies`.](https://github.com/jonkemp/gulp-useref/blob/93d64577de5dba6e460a029fd0f767668c5f163a/package.json#L22)