Don't delete dataset._meta (#204)

Don't delete dataset._meta and fix deprecation warning

Author: benmccann

samples/zoom-time.html

@@ -85,7 +85,7 @@
 					xAxes: [{
 						type: 'time',
 						time: {
-							format: timeFormat,
+							parser: timeFormat,
 							// round: 'day'
 							tooltipFormat: 'll HH:mm'
 						},

src/plugin.js

@@ -345,10 +345,6 @@ var zoomPlugin = {
 				}
 			});
 
-			helpers.each(chartInstance.data.datasets, function(dataset) {
-				dataset._meta = null;
-			});
-
 			chartInstance.update();
 		};