Set the default SSL provider to Secure Channel.

Current GVFS cache-servers use a CA that is not known to the CERTs being
distributed by GFW for OpenSSL. This causes errors such as the following
when talking to the cache-server:

* TLSv1.2 (OUT), TLS alert, unknown CA (560):
* SSL certificate problem: unable to get local issuer certificate
* Closing connection 0
curl: (60) SSL certificate problem: unable to get local issuer certificate

We should force the use of Secure Channel to avoid such issues.