Remove bootstrap plunker demo from README

Author: dimirc

README.md

@@ -4,7 +4,6 @@ AngularJS-native version of [Select2](http://ivaynberg.github.io/select2/) and [
 
 - [Demo](http://plnkr.co/edit/a3KlK8dKH3wwiiksDSn2?p=preview)
 - [Demo Multiselect](http://plnkr.co/edit/juqoNOt1z1Gb349XabQ2?p=preview)
-- [Bootstrap theme](http://plnkr.co/edit/QCwSM75ilH2Vh6D9aMA4?p=preview)
 
 Check [examples](https://github.com/angular-ui/ui-select/blob/master/examples).