Update image name to avoid conflict

Change the image name to prevent conflicting file names.  My project already had a file named arrow.png.  Perhaps I'm doing something wrong, I'm new to iOS development.  When I pulled in PullToRefreshView (via cocoapods) my arrows all changed to the PullToRefreshView arrow.

I originally forked btaylor/PullToRefreshView which is why there's a LICENSE file in this fork.