Explicitly list nlopt as a drake dependency
Explicitly list nlopt as a drake dependency

Fixes issue #2176

sheim/drake,billhoffman/drake,billhoffman/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake,sheim/drake,sheim/drake,sheim/drake,sheim/drake