terminal-notifier: update test

Test on Mavericks or earlier is as good as no test in late 2016. Whether
the test works on our bots on not (due to notification center problems),
the test needs an update.

Closes #5534.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/terminal-notifier.rb

@@ -29,6 +29,6 @@ class TerminalNotifier < Formula
   end
 
   test do
-    system "#{bin}/terminal-notifier", "-help" if MacOS.version < :yosemite
+    assert_match version.to_s, pipe_output("#{bin}/terminal-notifier -help")
   end
 end