dsl_test: unset HOMEBREW_DEVELOPER to avoid deprecation errors

Library/Homebrew/cask/test/cask/dsl_test.rb

@@ -69,11 +69,13 @@ describe Hbc::DSL do
     end
 
     it "may use deprecated DSL version hash syntax" do
-      test_cask = Hbc.load("with-dsl-version")
-      test_cask.token.must_equal "with-dsl-version"
-      test_cask.url.to_s.must_equal "http://example.com/TestCask.dmg"
-      test_cask.homepage.must_equal "http://example.com/"
-      test_cask.version.to_s.must_equal "1.2.3"
+      with_environment "HOMEBREW_DEVELOPER" => nil do
+        test_cask = Hbc.load("with-dsl-version")
+        test_cask.token.must_equal "with-dsl-version"
+        test_cask.url.to_s.must_equal "http://example.com/TestCask.dmg"
+        test_cask.homepage.must_equal "http://example.com/"
+        test_cask.version.to_s.must_equal "1.2.3"
+      end
     end
   end