Add 2.1.8 and 2.2.4 support

[erkde]

A new version of Ruby has been released, but trying to prepare a project for the upgrade fails on CI:

rbenv: version `2.2.4' is not installed
rbenv: version `2.2.4' is not installed
Build step 'Execute shell' marked build as failure
An attempt to send an e-mail to empty list of recipients, ignored.
Finished: FAILURE

Includes fix for CVE-2015-7551: Unsafe tainted string usage in Fiddle and DL

[alexmuller]

This will install rbenv-ruby-2.2.4 which I think needs to be added to our package archive first.

alexmuller@ci-slave-3:~$ apt-cache showpkg rbenv-ruby-2.2.4
N: Unable to locate package rbenv-ruby-2.2.4
N: Couldn't find any package by regex 'rbenv-ruby-2.2.4'

There are docs available for adding a new Ruby version but I think you'll need to grab somebody with access to Launchpad to help.

[erkde]

Ok, can try - I saw #323 after a search through past PRs to see how it was done, have added 2.1.8 which was also patched, for any apps using that.

[jamiecobbett]

Is this mergeable or should we close it?

[erkde]

We should probably add Ruby 2.3 here as well

[alexmuller]

#371 does Ruby 2.3.0

[erkde]

I don't have the fork for this PR any more more, so superseded by #373