fix(scrollspy): replace 'this' with '$scrollspy' in callbacks

Using 'this' in callbacks causes errors in IE, so replaced them with explicit '$scrollspy' instance reference.

should fix #1229 
I'd keep the `this` where the change is not needed, as it's more aligned with what directives will look like in the future (ES6, etc.). looks like only this line needs to be fixed for IE, no? Would also add a comment regarding the IE9 bug.