Tell CURL to use TLS instead of SSLv3.

SSLv3 has known security vulnerabilities.  Dropbox is going to disable
SSLv3 soon, so switch to TLS.
Tell cURL to verify the server's certificate.

Before this, the server's certificates were not verified, which is
insecure.  This change turns on certificate verification and uses the
"root" certificate list from the official Dropbox SDK for PHP.
Merge pull request #12 from cakoose/master

HTTPS security fixes: Use TLS instead of SSL v3; actually verify server certificate.