Update CONTRIBUTING.md.
Update CONTRIBUTING.md.

- Go 1.8 is the current minimum requirement.
- The main binary moved to cmd/traefik.
- Remove obsolete gox example.

vholovko/traefik,kekoav/traefik,vdemeester/traefik,marco-jantke/traefik,aantono/traefik,simbas/traefik,mmatur/traefik,dtomcej/traefik,ldez/traefik,thomasbach76/traefik,mihaitodor/traefik,thomasbach76/traefik,ldez/traefik,vholovko/traefik,simbas/traefik,FriggaHel/traefik,bparli/traefik,EmileVauge/traefik,adityacs/traefik,nmengin/traefik,timoreimann/traefik,FriggaHel/traefik,SantoDE/traefik,bparli/traefik,mmatur/traefik,mvdstam/traefik,vholovko/traefik,timoreimann/traefik,kekoav/traefik,FriggaHel/traefik,EmileVauge/traefik,bparli/traefik,dtomcej/traefik,timoreimann/traefik,mvdstam/traefik,containous/traefik,marco-jantke/traefik,timoreimann/traefik,FriggaHel/traefik,containous/traefik,marco-jantke/traefik,tanyadegurechaff/traefik,adityacs/traefik,mihaitodor/traefik,martinbaillie/traefik,mmatur/traefik,simbas/traefik,mildis/traefik,EmileVauge/traefik,SantoDE/traefik,aantono/traefik,vdemeester/traefik,aantono/traefik,nmengin/traefik,adityacs/traefik,vholovko/traefik,simbas/traefik,mmatur/traefik,vdemeester/traefik,mihaitodor/traefik,martinbaillie/traefik,marco-jantke/traefik,bparli/traefik,aantono/traefik,vdemeester/traefik,tanyadegurechaff/traefik,SantoDE/traefik,tanyadegurechaff/traefik,SantoDE/traefik,mildis/traefik,mvdstam/traefik,containous/traefik,ldez/traefik,containous/traefik,nmengin/traefik,mildis/traefik,nmengin/traefik,aantono/traefik,mvdstam/traefik,thomasbach76/traefik,mildis/traefik,kekoav/traefik,dtomcej/traefik,adityacs/traefik,thomasbach76/traefik,tanyadegurechaff/traefik,mihaitodor/traefik,dtomcej/traefik,kekoav/traefik,martinbaillie/traefik,ldez/traefik,EmileVauge/traefik,martinbaillie/traefik