docker version: restore top-level BuildTime to RFC3339Nano

Introduced by bff56f0 (cli/command/system: define struct for
formatting version).

In the "docker info" result, the Engine component's BuildTime should
be in time.ANSIC format, but the top level BuildTime field should use
time.RFC3339Nano.

Signed-off-by: Rob Murray <rob.murray@docker.com>

cli/command/system/version_test.go

@@ -58,7 +58,7 @@ func TestVersionFormat(t *testing.T) {
 			GoVersion:     "go1.8.7",
 			KernelVersion: "v1.0.0",
 			Experimental:  false,
-			BuildTime:     "Mon Jul  9 23:38:38 2018",
+			BuildTime:     "2018-07-09T22:38:38.000000000+00:00",
 			Components: []system.ComponentVersion{
 				{
 					Name:    "Engine",