Update to work with API v3

[MaxDiOrio]

Get the following error when attempting to connect. Guessing because the API is now at v3.

panic: interface conversion: interface {} is nil, not []interface {}

goroutine 1 [running]:
main.(*RancherAPI).containerUrl(0xc420165f48, 0x7ffeefbffa93, 0xc, 0x25, 0x0)
/private/tmp/rancherssh-20180815-46308-1kyckiw/rancherssh-1.1.3/main.go:189 +0x214c
main.(*RancherAPI).GetContainerConn(0xc420165f48, 0x7ffeefbffa93, 0xc, 0x1008d7d)
/private/tmp/rancherssh-20180815-46308-1kyckiw/rancherssh-1.1.3/main.go:249 +0x10f
main.main()
/private/tmp/rancherssh-20180815-46308-1kyckiw/rancherssh-1.1.3/main.go:303 +0x94