Quite literally two things, no more no less.

To install a specific version of bundler do:

gem install bundler -v x.x.x

Where x.x.x is the version to install. Probably well known, but I had to look it up. Then use that version run, instead of the the latest one you have installed:

bundle _x.x.x_ install

Those _ surrounding the version number are not a typo and it does look odd, but it works…