There was an ',' at the end of the line.
Now all _extract_*_formats routines return a list
Reported at https://github.com/rg3/youtube-dl/pull/7126#issuecomment-147327584
_form_hidden_inputs
Since they have changed again to the previous format, I've modified the regex to match both formats.
Unfortunately vimeopro.com doesn't support it yet.