Fix less error when running grunt

Updating `grunt-contrib-less` to `~0.9.0` fixes the following error when running `grunt` on the from zero-to-todos application example.

```
Running "less:development" (less) task
>> ParseError: Syntax Error on line 648 in bower_components/bootstrap/less/mixins.less:648:25
>> 647   padding-right: (@grid-gutter-width / 2);
>> 648   &:extend(.clearfix all);
>> 649 }
Warning: Error compiling LESS. Use --force to continue.
```