Travis: work around curl certificate issue on OS X
Travis: work around curl certificate issue on OS X

juhp/stack,Heather/stack,Blaisorblade/stack,sjakobi/stack,phadej/stack,narrative/stack,deech/stack,harendra-kumar/stack,Fuuzetsu/stack,deech/stack,mrkkrp/stack,luigy/stack,luigy/stack,MichielDerhaeg/stack,AndreasPK/stack,juhp/stack,AndrewRademacher/stack,AndrewRademacher/stack,Blaisorblade/stack,narrative/stack,achernyak/stack,achernyak/stack,sjakobi/stack,martin-kolinek/stack,martin-kolinek/stack,Heather/stack,mrkkrp/stack,Fuuzetsu/stack,anton-dessiatov/stack,phadej/stack,harendra-kumar/stack,AndreasPK/stack,anton-dessiatov/stack,MichielDerhaeg/stack