Uninstall ntlm-http and plist for failure of osx test.

It contains too old configuration that is autorequire. It will be
  removed at the RubyGems 3.1.0.

.travis.yml

@@ -97,6 +97,7 @@ env:
     os: osx
     compiler: clang
     before_install:
+      - gem uninstall ntlm-http plist -x --force
       - /usr/local/opt/openssl@1.1/bin/openssl version
 
   - &clang-8