docker-machine-driver-xhyve: make docker-machine recommended

If you have installed Docker components via Docker Toolbox (https://www.docker.com/products/docker-toolbox), whether manually or from Cask
this formula's dependency on docker-machine will cause a conflict.
This PR adds a `--without-docker-machine` option, which disables the dependency; the default behavior of the formula is unchanged.

Closes Homebrew/homebrew#48564.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Formula/docker-machine-driver-xhyve.rb

@@ -16,7 +16,7 @@ class DockerMachineDriverXhyve < Formula
 
   depends_on :macos => :yosemite
   depends_on "go" => :build
-  depends_on "docker-machine"
+  depends_on "docker-machine" => :recommended
 
   def install
     (buildpath/"gopath/src/github.com/zchee/docker-machine-driver-xhyve").install Dir["{*,.git,.gitignore}"]