Attempt to use ruby 2.3.1

The line I edited, to me, seems enough to make omnibus use ruby 2.3.1 but where the MD5 came from was unknown to me.

So I'd appreciate a pointer here where I could find it, as it didn't match with the md5 I got after downloading the tar.bz from ruby-lang.org

Closes #1377 (closed)

/cc @connorshea @DouweM