Added if statement to allow string 'false' in the disabled

This makes the $observer check for `value === 'false'` so if the disable equivocates to false and angular places the string 'false' in the attribute, the $observer will still fucntion

```html
<fieldset data-ng-repeat="comp in complications">
    <label>{{ comp.spec_field }}:</label>
    <select ui-select2
                width='100%'
                minimum-results-for-search='-1'
                data-placeholder='Chose an option'
                data-ng-model="select[$index]"
                data-disabled="{{ !selects[comp.spec_field] }}"
                data-ng-change="calculateSelects($index)">
            <option value=""></option>
            <option ng-repeat="(_, spec) in selects[comp.spec_field]" value="{{ spec.guids }}">{{ spec.title.join(', ') }}</option>
    </select>
</fieldset>
```