look for either 'nimrod' or 'nim' as the compiler

I just installed 0.9.6 of nim from source, then installed the plugin via Vundle. After opening a .nim file, vim was complaining about not finding the compiler in the path. On line 9 of the `autoload/nim.vim` script it was only looking for a binary named `nim` in the path. I updated it to find either `nimrod` or `nim` in the path and it's now working.

I'm new to Nim but it appears the old binary was `nim`? Or maybe that's the new name? Either way 0.9.6 when built as per the website produces a binary named `nimrod`, so the plugin now looks for it.