Use process.platform variable instead of os.platform which is a function now.

I tried to use cult on windows but it didn't work. It said:

    [cult] Error, can't find gulp command

I poked around a bit and noticed that os.plaftorm is a function not a variable. I noticed that there is process.plaform variable that has value win32 for all windows systems (my 64 bit windows also) so I used it instead. Now cult works for me.