Fixed imageLoaderLimit

Currently jobsInProgress was not incremented after adding a job. So it
has gone into the - range and was like unlimited jobs.

src/imageLoader.js

@@ -162,6 +162,7 @@ function completeJob( loader, job, callback ) {
     if ( (!loader.jobLimit || loader.jobsInProgress < loader.jobLimit) && loader.jobQueue.length > 0) {
         nextJob = loader.jobQueue.shift();
         nextJob.start();
+        loader.jobsInProgress++;
     }
 
     callback( job.image );