[Linux] Kernel panic if nfs server isn't running

[glinton]

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x878118]

goroutine 1 [running]:
github.com/nanobox-io/nanobox/util/provider.DockerMachine.Valid(0xc4203007c0, 0xe, 0xc42030c1a8, 0xa68301, 0x7)
	/home/travis/gopath/src/github.com/nanobox-io/nanobox/util/provider/dockermachine.go:80 +0xa98
github.com/nanobox-io/nanobox/util/provider.(*DockerMachine).Valid(0xfede68, 0xfede68, 0x0, 0x0, 0x965640, 0xc4202fe110)
	<autogenerated>:1 +0x35
github.com/nanobox-io/nanobox/util/provider.Valid(0xc420300390, 0xb, 0xa6831f, 0x7, 0x0)
	/home/travis/gopath/src/github.com/nanobox-io/nanobox/util/provider/provider.go:64 +0x18a
main.main()
	/home/travis/gopath/src/github.com/nanobox-io/nanobox/main.go:86 +0x4dd