zsh: fix test

Better tests are certainly welcome, but keep in mind that the source is
not available to `brew test`. If `make check` is necessary it should be
done as part of the build.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/zsh.rb

@@ -34,7 +34,7 @@ class Zsh < Formula
   end
 
   def test
-    system "HOME=\"#{prefix}\" make check"
+    system "#{bin}/zsh --version"
   end
 
   def caveats; <<-EOS.undent