fix(bootstrap) Show placeholder on disabled multiple

Ensure placeholder is still visible when select is disabled.

Closes #1667

Author: Jefiozie

src/bootstrap/select-multiple.tpl.html

@@ -9,7 +9,6 @@
            class="ui-select-search input-xs"
            placeholder="{{$selectMultiple.getPlaceholder()}}"
            ng-disabled="$select.disabled"
-           ng-hide="$select.disabled"
            ng-click="$select.activate()"
            ng-model="$select.search"
            role="combobox"