Call OmniSharpServer correctly in Cygwin

* Utils.OnWindows is false for cygwin. Add OnCygwin test, and exclude OnCygwin from calling Omnisharp with Mono as well.
* Update Omnisharp submodule to pick up recent Cygwin support, and pass the needed command argument to enable it.

This is #10 and #11 in https://github.com/Valloric/YouCompleteMe/pull/866