Update guzzlehttp/guzzle to v5
Update guzzle utils usage
Update client internals to work with RingPhp

The client can no longer rely on different adapter/parallel adapter for
batching requests.
Remove parallel adapter supporting code
Update readme
Update guzzle links
Remove reference to Ubuntu 12.04
Remove unused use statement
Allow failures on HHVM

Waiting for `Closure::bind` support to be added.
Merge pull request #16 from graze/guzzle-5

Updates to support GuzzleHTTP 5 and RingPHP handlers