brew-cask 0.60.1

Over at Homebrew we've accidentally caused an edge-case problem for some
Caskroom users. We recently merged Homebrew/legacy-homebrew@915ac06d
which is much stricter on empty installations, failing during install rather than
just shouting about it in the audit.

We'd prefer not to revert the change there because it's quite a positive change
that's much more obvious about installation problems, given installations
shouldn't be empty. There's also the likelihood that if we revert, wait 3 months
to add the check back, someone out there will wait 4 months before upgrade and we'll
run into this again.

However, this causes some issues with Cask because when Mike rewrote the formula
he left `def install` an empty hole, which was fine at the time.
Anyone who hasn't updated between December 9th 2015 and January 9th 2016 will now
run into a fatal error on upgrading the Cask formula, as seen in: Homebrew/legacy-homebrew#47929

I was wondering if the Cask would be kind enough to tag one very last bug fix release,
or even retag the existing tag and formula with a similar fix to this.
If you're agreed and prefer to do this yourself you can ignore the PR but wanted
to explain the background.

Author: DomT4

brew-cask.rb

@@ -1,17 +1,21 @@
 class BrewCask < Formula
   homepage "https://github.com/caskroom/homebrew-cask/"
-  url "https://github.com/caskroom/homebrew-cask.git", :tag => "v0.60.0"
+  url "https://github.com/caskroom/homebrew-cask.git", :tag => "v0.60.1"
 
   depends_on :ruby => "2.0"
 
+  UNINSTALL_MSG = <<-EOS.undent
+    You must uninstall this formula. It is no longer needed to stay up to date,
+    as Homebrew now takes care of that automatically.
+  EOS
+
   def install
+    (buildpath/"UPGRADE").write UNINSTALL_MSG
+    prefix.install "UPGRADE"
   end
 
   def caveats
-    <<-EOS.undent
-      You can uninstall this formula as `brew tap Caskroom/cask` is now all that's
-      needed to install Homebrew Cask and keep it up to date.
-    EOS
+    UNINSTALL_MSG
   end
 
   test do