Updating index.js to use Node v4.3

As mentioned in AWS documentation to migrate from Node v0.10 to v4.3, context functions would need to be updated to use the new nomenclature.

ref: http://docs.aws.amazon.com/lambda/latest/dg/nodejs-prog-model-using-old-runtime.html#transition-to-new-nodejs-runtime

This commit will update the `context` functions of:

* context.fail(object)
* context.succeed()

To Node v4.3 format following what is suggested by the documentation provided by AWS.

Author: Cristiano Diniz da Silva

index.js

@@ -48,7 +48,8 @@ exports.handler = function(event, context) {
 		});
 	}, function(err, images) {
 		if (err) {
-			context.fail(err);
+			context.callbackWaitsForEmptyEventLoop = false;
+			callback(err, 'Failed result');
 		} else {
 			var resizePairs = cross(CONFIG.sizes, images);
 			async.eachLimit(resizePairs, CONFIG.concurrency, function(resizePair, cb) {
@@ -70,9 +71,11 @@ exports.handler = function(event, context) {
 				});
 			}, function(err) {
 				if (err) {
-					context.fail(err);
+					context.callbackWaitsForEmptyEventLoop = false;
+					callback(err, 'Failed result');
 				} else {
-					context.succeed();
+					context.callbackWaitsForEmptyEventLoop = false;
+					callback(null, 'Success message');
 				}
 			});
 		}