Fix "make test"

Didn't work previously due to external "should" library being required
instead of the local (included) "should" library.

Full test suite ("make" or "make run-tests") still doesn't work, which
appears to be a deeper issue.

Some changes taken from https://github.com/pghalliday/socket.io-client/commit/c9f0d5392732dc6bd15748b16fef1d5dd71f04d7