[4.1] Added more secure peer verification

This is the better-understood version of #281 and should fully address #183 for-realz this time. :)

In the event Facebook changes their TLS/SSL certificate authority, all implementations of this version of the PHP SDK will break. It sounds scary, but this is hella more secure.

And put this on as a TODO for **November 10, 2031**: Update the root CA, it has expired. :)

Learned a lot on this PR. Thanks to @zerkms for brining up the original issue and @ngyikp for helping me better understand this!

I also uncovered two bugs with the Guzzle implementation that I fixed in this PR as well.

1. There were some cases when Guzzle would throw an exception and it would not get caught and rethrown properly.
2. The Guzzle-formatted response array headers are multidemensional and were not being handled properly.