pypy compat: Implement SocketDuckForFd's drop/reuse
travis: Run tox in verbose mode
travis: Print tox and pip logs in case of failure
travis: Upgrade dependencies
travis: Install ZeroMQ development files
pypy compat/tests: Set correct size of Psycopg2 db pool
pypy compat: Make db pool tests clean after themselves
tests: Handle pyzmq CFFI backend correctly

The inconsistency between Cython and CFFI backend is fixed in pyzmq
master, see:

* https://github.com/zeromq/pyzmq/issues/523
* https://github.com/zeromq/pyzmq/pull/524
pypy compat