Investigate removal of `override` for docker/docker in Gopkg.toml

[allisaurus]

As pointed out in #980 , the github.com/docker/docker [[override]] clause is currently targeting a revision that is several minor versions behind latest*. We should investigate whether we can remove this override or, minimally, update the targeted revision.

*This override was originally put in place to solve some build time dependency resolution errors that were being seen while adding support for mandatory variables.

[allisaurus]

so naively updating to the lastest commit in moby/moby results in some build errors in ecs-cli/modules/cli/local. Looks like some Methods were added to the the Client interface? Example:

# github.com/aws/amazon-ecs-cli/ecs-cli/modules/cli/local
ecs-cli/modules/cli/local/down_app.go:35:19: cannot use client (type *"github.com/aws/amazon-ecs-cli/ecs-cli/vendor/github.com/docker/docker/client".Client) as type "github.com/aws/amazon-ecs-cli/ecs-cli/modules/cli/local/network".LocalEndpointsStopper in argument to "github.com/aws/amazon-ecs-cli/ecs-cli/modules/cli/local/network".Teardown:
	*"github.com/aws/amazon-ecs-cli/ecs-cli/vendor/github.com/docker/docker/client".Client does not implement "github.com/aws/amazon-ecs-cli/ecs-cli/modules/cli/local/network".LocalEndpointsStopper (wrong type for ContainerRemove method)
		have ContainerRemove("context".Context, string, "github.com/aws/amazon-ecs-cli/ecs-cli/vendor/github.com/docker/docker/api/types".ContainerRemoveOptions) error
		want ContainerRemove("github.com/aws/amazon-ecs-cli/ecs-cli/vendor/golang.org/x/net/context".Context, string, "github.com/aws/amazon-ecs-cli/ecs-cli/vendor/github.com/docker/docker/api/types".ContainerRemoveOptions) error
ecs-cli/modules/cli/local/up_app.go:130:32: cannot use docker.NewClient() (type *"github.com/aws/amazon-ecs-cli/ecs-cli/vendor/github.com/docker/docker/client".Client) as type "github.com/aws/amazon-ecs-cli/ecs-cli/modules/cli/local/network".LocalEndpointsStarter in argument to "github.com/aws/amazon-ecs-cli/ecs-cli/modules/cli/local/network".Setup:
	*"github.com/aws/amazon-ecs-cli/ecs-cli/vendor/github.com/docker/docker/client".Client does not implement "github.com/aws/amazon-ecs-cli/ecs-cli/modules/cli/local/network".LocalEndpointsStarter (wrong type for ContainerCreate method)
		have ContainerCreate("context".Context, *container.Config, *container.HostConfig, *"github.com/aws/amazon-ecs-cli/ecs-cli/vendor/github.com/docker/docker/api/types/network".NetworkingConfig, string) (container.ContainerCreateCreatedBody, error)
		want ContainerCreate("github.com/aws/amazon-ecs-cli/ecs-cli/vendor/golang.org/x/net/context".Context, *container.Config, *container.HostConfig, *"github.com/aws/amazon-ecs-cli/ecs-cli/vendor/github.com/docker/docker/api/types/network".NetworkingConfig, string) (container.ContainerCreateCreatedBody, error)
make: *** [bin/local/ecs-cli] Error 2