Increase minimum libcurl version for curl_global_sslset
Increase minimum libcurl version for curl_global_sslset

Although curl_global_sslset() is present in 7.56.0, it contained several bugs
which prevent it from being used as currently implemented in PycURL.
Specifically, older versions of curl_global_sslset did not work properly when
-1 was passed as the first argument.  Also, it did not always return the
available backends.  Rather than attempt to handle these quirks in PycURL, just
increment the minimum libcurl version.

Relevant bugs:
https://curl.haxx.se/bug/?i=3346
https://curl.haxx.se/bug/?i=2499

pycurl/pycurl,pycurl/pycurl,pycurl/pycurl