brew.rb: handle cask autoinstall when cask tapped.

Otherwise it would assume if `cask` was tapped it was also installed.

Library/brew.rb

@@ -162,21 +162,27 @@ begin
       Tap.fetch("Homebrew", "services")
     end
 
+    command_installed = false
+
     if possible_tap && !possible_tap.installed?
       possible_tap.install
+      command_installed = true
+    end
 
-      if cmd == "cask"
+    if cmd == "cask" && (brew_cask = Formulary.factory("brew-cask")) \
+       && !brew_cask.installed?
       require "cmd/install"
-        brew_cask = Formulary.factory("brew-cask")
       Homebrew.install_formula(brew_cask)
+      command_installed = true
     end
 
+    if command_installed
       exec HOMEBREW_BREW_FILE, cmd, *ARGV
-    end
-
+    else
       onoe "Unknown command: #{cmd}"
       exit 1
     end
+  end
 
 rescue FormulaUnspecifiedError
   abort "This command requires a formula argument"