Disable futures for cygwin

Cygwin testing takes a while and it doesn't seem to be much benefit of running
futures on cygwin.
Merge pull request #701 from ronawho/cygwin-futures

Disable futures for cygwin

Cygwin testing takes a while and it doesn't seem to be much benefit of running
futures on cygwin.

Future feature request from Brad that seems like a good idea: 

"I had a feature request thought for futures -- curious what you think. I think it'd be nice if, in addition to "futures on" and "futures off" there was a middle setting in which futures would be tested if they had a .skipif file. That way, if one had a future that was cygwin-specific, say, they could add a skipif to skip it on everything other than cygwin; yet the test would still be tested on cygwin (without testing all 500 other futures which are presumably independent of cygwin vs. linux64)."