added node 6 to travis
added node 6 to travis

ethereumjs/ethereumjs-tx,michbil/ethereumjs-tx,ethereumjs/ethereumjs-tx,ethereumjs/ethereumjs-vm,ethereum/ethereumjs-tx,ethereumjs/ethereumjs-vm,ethereumjs/ethereumjs-vm,ethereumjs/ethereumjs-vm,ethereum/ethereumjs-vm,ethereumjs/ethereumjs-vm