add two declarations to support older versions of numpy (<1.7)

should fix #10, sorry for that. It seems numpy.i is not as backward compatible as I thought. I tested with numpy 1.6, 1.7 and 1.9 (all on OS X), I could reproduce the error with 1.6, and it works for all of them with the fix. numpy 1.5 did not compile on my machine right now, so I couldn't test (will try again, but wanted to push this first).