The most recent release of JRuby installed on the containers is 1.7.19, released 9 months ago. JRuby 9k is missing, but has been out for 3 months. The current stable version of JRuby 9k (jruby-9.0.3.0) cannot currently install gems on CircleCI.
I have sent questions to support related to this, but haven’t really received any clear answers.
We just shipped jruby-9.0.1.0 last night. Still need to update our doc, but you can already specify it in your circle.yml.
@kimh thank you, glad to hear. Is there anything I can do to get jruby-9.0.3.0 to work in the meantime?
Ah, we created the new container image about a week ago but jruby-9.0.3.0 was released 6 days ago 
You can still use 9.0.3.0 with the following circle.yml.
machine:
ruby:
version: jruby-9.0.3.0
but this makes your build a bit slower because it basically does rvm install jruby-9.0.3.0 in every build.
We’ll pre-install jruby-9.0.3.0 in the next container image release.
@kimh yes, that’s what I have right now, but I’m unable to install any gems.
@mitchellhenke can you share some output or build URLs of what happens when you try to install gems?
@mitchellhenke I did some digging and it looks like the latest version of jruby on CircleCI is affected by this bug.
I was able to get this to work by updating to 9.0.4.0:
ubuntu@box1309:~/api$ which gem
/home/ubuntu/.rvm/rubies/jruby-9.0.4.0/bin/gem
ubuntu@box1309:~/api$ gem install bundler --pre
Fetching: bundler-1.10.6.gem (100%)
Successfully installed bundler-1.10.6
1 gem installed
It looks like it has been fixed in the latest 9.0.4.0 and should be fixed for sure once 9.0.4.0 is released. We will include this version once it has been released in our image. In the meantime you can workaround this by adding the following to your circle.yml file.
machine:
ruby:
version: jruby-9.0.4.0
Can you five this a try and let me know if it works for you?
FYI: We’ve released a new container image that preinstalls jruby-9.0.3.0 and 9.0.4.0.
When jruby-9.0.5.0 will be available?