Update README.md.
Fixes #72.
Use Travis-CI to run tests on commit.

Unfortunately, because Travis appears to use ZMQ internally, they have
a custom build of zmq3 that doesn't have proper conflict rules, and
fails to allow installation of libmzq-dev (2.x). So we're only testing
against zmq 3.x for now.
Support EINVAL as syscall.EINVAL.

Since syscall.EINVAL != gozmq.EINVAL we can support only one.

syscall.EINVAL is what actually worked before and this is what the tests
have been using, so it is the most backwards-compatibile.

This commit changes gozmq.EINVAL from a zmqErrno to a syscall.Errno.