Update image name to avoid conflict

[zjacobson]

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.