Appveyor: Use builtin bundler for ruby-2.6
Appveyor: Use builtin bundler for ruby-2.6

This reverts commit ff1e22509c0cdc77ad488f5b18feb360829e8650.

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