Update gulp-util dependency

Depending on gulp-util 2.x breaks installation for users with recent releases of gulp-util; e.g.

```
npm install                                                                                                                              
npm WARN unmet dependency node_modules/gulp-coveralls requires gulp-util@'~2.2.0' but will load
npm WARN unmet dependency node_modules/gulp-util,
npm WARN unmet dependency which is version 3.0.1

```