Remove unused stream-assert-gulp

Depcheck gives this output:

```
❯ depcheck
Unused Dependencies
* debug
* gulp-ruby-sass
* jade
* node-bourbon
* stream-assert-gulp
* yargs

Unused devDependencies
* gulp-flatten
* gulp-jshint
* jscs
* karma
* karma-chai
* karma-mocha
* karma-phantomjs-launcher
* mocha
* sinon
* tiny-lr
```

Some of them can be also removed.