Update documentation to use Ruby 2.3.1

GitLab is using Ruby 2.3.1 now on master, I figure the documentation should also be updated.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4948

cc: @zj @jacobvosmaer-gitlab

Status changed to merged

mentioned in commit 0ca72659

Thanks @connorshea ! Now excuse me while I go and install Ruby 2.1.3 on my computer.

You might already have that one ;) You should try rvm install 2.3.1

@jacobvosmaer-gitlab I think you'd be missing some security patches if you did that :P

Lol it took me this long to see the typo '2.1.3' vs '2.3.1'

Luckily, I don't do as I say

% ruby --version
ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-darwin15]