Looks like bundler 2 has just been released, which requires rubygems 3 which we don't have anywhere in Evergreen.
The CI is failing thusly:
[2019/01/03 13:37:22.159] + gem install bundler
[2019/01/03 13:37:22.159] ERROR: Error installing bundler:
[2019/01/03 13:37:22.159] The last version of bundler (>= 0) to support your Ruby & RubyGems was 1.17.3. Try installing it with `gem install bundler -v 1.17.3`
[2019/01/03 13:37:22.159] bundler requires RubyGems version >= 3.0.0. The current RubyGems version is 2.7.6. Try 'gem update --system' to update RubyGems itself.
[2019/01/03 13:37:22.163] ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
For the time being we should probably just request bundler < 2.
Our travis config updates rubygems prior to installing bundler, and should therefore get rubygems 3 and bundler 2.