Fix URL

Some OS need PIL to import Image, like Arch Linux.
Fix function call, wrong name with url.
Hum, my commit miss explication.
The pep8 is ok.
On this commit, https://github.com/joshvillbrandt/GoProController/commit/9543ef4a7dc3ccb67be8547fdf866a869efc9941#diff-6d278679bdd74e9daf073f724a2052a5, you change previewURL to _previewURL
But the definition of this function was _commandUrl