Delegate to docker-machine from wrapper

Fix an issue with the bash __docker-machine-wrapper function where
commands other than `use` would result in an error.

Signed-off-by: David Schlosnagle <schlosna@gmail.com>

contrib/completion/bash/docker-machine-wrapper.bash

@@ -47,7 +47,7 @@ EOF
         esac
     else
         # Just call the actual docker-machine app
-        $(which docker-machine) "$@"
+        command docker-machine "$@"
     fi
 }