brewRenderer: Capture event properties while still in event loop.

React [pools events](https://github.com/facebook/react/issues/2850),
which means that once the lambda runs later on the properties we want
to read will be null. Instead, we capture the properties in the event
loop.

client/homebrew/brewRenderer/brewRenderer.jsx

@@ -58,8 +58,9 @@ const BrewRenderer = createClass({
 	},
 
 	handleScroll : function(e){
+		let target = e.target;
 		this.setState((prevState)=>({
-			viewablePageNumber : Math.floor(e.target.scrollTop / e.target.scrollHeight * prevState.pages.length)
+			viewablePageNumber : Math.floor(target.scrollTop / target.scrollHeight * prevState.pages.length)
 		}));
 	},