FIX dropdown not showing

UI bootstrap dropdown directives are no more class directives:
https://github.com/angular-ui/bootstrap/blob/master/src/dropdown/dropdown.js#L119
https://github.com/angular-ui/bootstrap/blob/master/src/dropdown/dropdown.js#L128

Their directive definition object (DDO) has no 'restrict' property. They are therefore restricted to 'attribute and element' directives. From the angular.js doc:

````
Note: When you create a directive, it is restricted to attribute and elements only by default. In order to create directives that are triggered by class name, you need to use the restrict option.
````