Active-campaign-rails gem not installing when running bundle install

In our application we use the Active Campaign gem for rails active-campaign-rails. In the dev environment on our machines bundle install works as expected with no errors and all tests pass. However, on the Circle CI build when bundle install is run, it gives this error: Gem::InstallError: active-campaign-rails requires Ruby version >= 2.0.0.
From research I have done, this could be because of differing ruby versions and in our project we do not have a .ruby-version file. However, since this works on my dev environment I am not sure the steps I need to take to get it to work on Circle.

Thanks for your help in advance!