travis: Build with PyPy

This includes few changes that just update dependencies and at least one
that's necessary for the tests to even run.
pypy compat: Implement SocketDuckForFd's drop/reuse
pypy compat: Fix kqueue hub

PyPy (at least 2.2.x) raises IOError instead of OSError
tests: Make greenthread tests clean after themselves
pypy compat: Fix ssl wrap_socket test

Used to fail with "error: [Errno 9] Bad file descriptor"
pypy compat: Fix various tests
pypy compat: Fix greenio test_del_closes_socket test
pypy compat: Fix greenio tests
pypy compat: Fix refcount tests