Appveyor: Use bundled bundler when running on ruby-2.5.
Appveyor: Use bundled bundler when running on ruby-2.5.

larskanis/rubyinstaller2,oneclick/rubyinstaller2,larskanis/rubyinstaller2,oneclick/rubyinstaller2,oneclick/rubyinstaller2,larskanis/rubyinstaller2