Replaced fancy C++11 lambda with old style static function wrap in Downloader

Replaced fancy C++11 lambda with old style static function wrap in Downloader

The C++11 lambda would work, but the lambda has to be cast to a
C-style function pointer first, cURL however doesn't provide function
pointer definitions for quick and easy casting and since it takes
arguments as va_arg it crashes and burns when passing in a raw lambda.
Old style _wrap is simply easier then casting to the proper function type.