Downgrade bundler as per bundler/bundler#6537
Downgrade bundler as per bundler/bundler#6537

ref: https://github.com/bundler/bundler/issues/6537
kachick/opal,kachick/opal,Mogztter/opal,Mogztter/opal,kachick/opal,opal/opal,opal/opal,Mogztter/opal,kachick/opal,opal/opal,opal/opal,Mogztter/opal