Fix CI tests on darwin

Vagrantfile

@@ -35,7 +35,6 @@ end
 def packages_darwin
   return <<-EOF
     ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
-    brew install caskroom/cask/brew-cask
     brew cask install osxfuse
   EOF
 end

run_integration_tests.go

@@ -97,7 +97,6 @@ func (env *TravisEnvironment) Prepare() {
 	if runtime.GOOS == "darwin" {
 		// install the libraries necessary for fuse
 		run("brew", "update")
-		run("brew", "install", "caskroom/cask/brew-cask")
 		run("brew", "cask", "install", "osxfuse")
 	}