Use modern .ruby-version filename

Get rid of .rvmrc in gitignore and use .ruby-version from now on.
Gives everyone freedom to choose RVM, rbenv and friends.
Create version 2.4.0.beta

I am releasing a beta version first, because the gem hasn't seen
updates in a while. It's just based on master. When nothing breaks
in a week, I will release 2.4.0 proper.