Add a default timeout & connect_timeout to Guzzle

[imjoehaines]

Goal

By default Guzzle has no timeout, meaning it waits indefinitely for requests to finish. This PR applys a very conservative default timeout of 15 seconds to avoid blocking scripts forever*

This is only applied when Bugsnag constructs the Guzzle instance used; if one is provided in the constructor, no timeout will be added (though it can, of course, provide its own timeout value)

Design

To avoid yet more Guzzle version checks, I've consolidated them into a new GuzzleCompat class

Testing

Some basic unit tests check that a timeout is applied by makeGuzzle. Testing the timeout itself is a bit out of scope