Ensure that variable assigning declarations are single line

I am working through a code style cleanup effort using [jscs](https://github.com/mdevils/node-jscs) rules. 

You can see the script and rules in: https://github.com/rwjblue/ember.js/compare/code-style-rules

------

As referenced in #5351, this tweaks the rule to require all assignments to be in a `var` declaration by itself.