test_gpg: tweak rmtree usage

Swats away this annoying warning when the test is skipped due to GPG being
unavailable:

```
  1) Skipped:
GpgTest#test_create_test_key:
GPG Unavailable

Error:
GpgTest#test_create_test_key:
NoMethodError: undefined method `rmtree' for nil:NilClass
    /usr/local/Library/Homebrew/test/test_gpg.rb:11:in `teardown'
```

Closes #675.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Library/Homebrew/test/test_gpg.rb

@@ -7,10 +7,6 @@ class GpgTest < Homebrew::TestCase
     @dir = Pathname.new(mktmpdir)
   end
 
-  def teardown
-    @dir.rmtree
-  end
-
   def test_create_test_key
     Dir.chdir(@dir) do
       with_environment("HOME" => @dir) do
@@ -18,5 +14,7 @@ class GpgTest < Homebrew::TestCase
         assert_predicate @dir/".gnupg/secring.gpg", :exist?
       end
     end
+  ensure
+    @dir.rmtree
   end
 end