Cygwin compatibility bugfix

I tried to run apiary2postman on windows under cygwin. Due to check with command "which" I cannot use standard command line.

NamedTemporaryFile under cygwin uses Unix-like folders tree, so temp folder is usually located under /tmp. Unfortunately when you start process via Popopen it runs in windows environment where folder /tmp is not available.

I don't know why author used NamedTemporaryFile but on the Windows it works perfectly without it.