cannot build at macOS 10.12

[antwal]

Build error:

github.com/getlantern/systray

systray_darwin.m:132:22: error: use of undeclared identifier 'NSControlStateValueOn'
systray_darwin.m:134:22: error: use of undeclared identifier 'NSControlStateValueOff'

pls fix retro compatibility

[antwal]

I'm testing, I'll answer you on the PR