Test on 2.3.1

[connorshea]

I don't know if 2.3.1 with OS X works on Travis, but since we'd like to upgrade to Ruby 2.3 at GitLab it'd be nice to know vmstat supports 2.3 :)

Alternatively, use a matrix and only test it on Linux?

[threez]

The problem was/is that there is no rvm binary package available for os x yet. They turn that flag on by default. The linux worker had no problems as the code doesn't have. I will have a look at the matix exclusion.

[threez]

Doing this is actually kind of hard. I'm using ruby-head right now, since it should be supported but even that doesn't work...

[threez]

im running the tests against head now.