Upgrade react/stream to 0.7

[bzikarsky]

In regards to #10: Currently react/stream is fixed to 0.4.*. The most recent version is 0.7. We cannot upgrade due to a number of BC breaks between 0.4 and 0.7.

The breaks are documented at https://github.com/reactphp/stream/releases, but react-greaman definitly suffers from:

• 0.6.0, 0.7.0: Add DuplexResourceStream and deprecate Stream, Remove deprecated Stream class, use DuplexResourceStream instead
• 0.5.0: Remove custom (undocumented) full-drain event from Buffer