I am in an older Rails project that has a Gemfile. I tried to add a gem to the Gemfile and bundle install but got an error:
Bundler could not find compatible versions for gem "bundler":
  In Gemfile:
    rails (= 3.0.0) ruby depends on
      bundler (~> 1.0.0) ruby
  Current Bundler version:
    bundler (1.1.5)
This Gemfile requires a different version of Bundler.
The version of Rails it's using requires bundler ~>1.0.0 but I have 1.1.5 installed and am using it for my other projects. Usually I would use bundle exec ... but since this is bundler we are talking about, it's a little more complicated than that. How can I add a gem to my Gemfile and run bundle install while using the version of bundler that it requires?