Bug#876058: xvfb-run does not use /bin/bash in shebang line

[Rian Hunter <rian@alum.mit.edu>]

>> This is especially broken when "#/bin/sh" links to "dash" (now the
>> default shell on debian systems).
> 
> That appears to be true, however.  See #390433[2].

Thanks for the pointer. I'll see about getting a proper fix in the dash package.