Using the system CA bundle by default when possible

This commit updates Guzzle to use the CA bundle packaged on the
system by default when possible. cURL will use the system CA
bundle without issue, however, PHP 5.5 and lower will still
default to the bundled CA cert. You can now pass "bundled" to
use the cacert file bundled with Guzzle.

@klausi / @catch56: I'd love feedback on this if you have time.

I've gotten feedback from some sysadmins at work, and they'd prefer if Guzzle defaulted to the system's CA bundle. That seemed easy enough when Guzzle just used cURL, but now that Guzzle also uses PHP's stream wrapper, it gets a bit complicated. PHP version < 5.6 do not know where the system's default bundles are located, so there's two options: find it on disk or use a Guzzle bundled cert by default.

Edit: I've updated the PR to scan for common ca bundles on disk when one is not present by default for the PHP stream wrapper when using PHP < 5.6. I've removed the bundled CA bundle and am now throwing a very descriptive error if this problem is encountered.