Switched from 'Windows NT' to 'Win' as OS identifier (#79)

Switched to using the new identifier 'Win'
No point in keeping the old one.

Tested this on the sandbox server:
http://mozmill-sandbox.blargon7.com/#/l10n/reports?branch=All&platform=All&from=2013-12-31&to=2014-01-03

@whimboo please have a look.