We're not interested in minor versions right now

We're only interested in major backwards compatability
for the moment. It's too much of an issue to keep up
with minor version bugs in MRI.
:+1: for removal of 2.1.0 and 2.1.1 .

I think that `ruby-head` is the next developed MRI ruby which is probably the work towards 2.2.0 . Besides `ruby-head` we should also state `2.1` here to have the latest ruby in the 2.1 series (which is 2.1.3 at this moment).