This is required since bundler support seems to be broken right now on
the default "image" of jruby-head on Travis. If we install it explicitly
(or update it to the latest version on all other Rubies), we will be
fine though.
We wnumerate exact Ruby versions in .travis.yml. This is required for
at least Ruby 2.4 which isn't currently recognized as a fuzzy match in
Travis' RVM version. When specifying it as an exact version, it will
install it though.
We specify all the other versions too since this makes our tests more
reproducable over time.