fix: correctly check for provided dependencies

My current setup inside `browserify` looks like this:

```js
var angular = require('angular')
var ngTimeRelative = require('ng-time-relative')
var moment = require('ng-time-relative/node_modules/moment')

var app = angular.module('woop', [])
app.constant('moment', moment)
ngTimeRelative(app)
```

This way in the browser `if (moment) {`(line 30) fails with "Uncaught ReferenceError: moment is not defined ".

Correctly checking for the undefinedness of the provided modules fixes that.