Allow to completely skip libCURL initialization in Ppconsul (#65)

Upgrade to Catch v2.13.6 to support Apple M1

Update copyright year to 2020

fix compilation issue from missing cassert include statement

Support ppconsul::kw::enable_stop for libcurl <= 7.31.0 (e.g. RedHat 7)

using deprecated CURLOPT_PROGRESSFUNCTION/CURLOPT_PROGRESSDATA inside
a #ifdef