backends do not load in proper order

[zzjjbb]

This line has a comment # Iterate in reverse order of preference. However, it loads them with some random order, because the object to iterate is a set, which doesn't keep the order. In my Linux computer, when I run import arrayfire as af, the function will be called and it loads "" first, and due to err!=0, it directly returns and not loads anything.

By the way, I simply change {"cpu", "opencl", "cuda", ""} to ["cuda"] and I can import arrayfire with no error.

[syurkevi]

This is a mistake in the dev branch, the master branch should still have the correct code. Thank you for catching that! I'll make sure to address if we decide merge dev into master. There are a few incoming changes (related to library.py) that need to go into effect before we will be looking at the dev branch.