Add Ruby 2.4 and ruby-head in .travis.yml

[junaruga]

I want to add ruby-2.4 and ruby-head in .travis.yml.

Ruby 2.4

I added "2.4.0", not "2.4".
Because last month I tried "2.4" in .travis.yml, it was not available.
I can not find official document for that.

ruby-head

I want to add ruby-head as allow_failures in .travis.yml.
I think this is useful.
Because we can prepare before next version Ruby 2.5 release.
Though it might not be useful until the actual preview release.
We can support the next version as faster.

We can see this kind of logic in rails, rspec and cucumber and etc.
I think that is the reason why rails and rspec can support new version Ruby as faster.

https://github.com/rails/rails/blob/master/.travis.yml
https://github.com/rspec/rspec-core/blob/master/.travis.yml
https://github.com/cucumber/cucumber-ruby/blob/master/.travis.yml
https://github.com/sparklemotion/nokogiri/blob/master/.travis.yml

Is it possible to add it?

Thanks.

[floehopper]

Thanks. I'd be happy to accept this PR if you split it up into multiple commits as follows:

1. Convert all existing MAJOR.MINOR Ruby versions to latest MAJOR.MINOR.PATCH as suggested in rvm: 2.4 installation fails travis-ci/travis-ci#7077. Please reference this issue in the commit message.
2. Additions for Ruby v2.4.0 - it needs to be added here, here, and here. Your PR missed the third of these, i.e. the one for Test::Unit.
3. Additions for ruby-head. I think this should be added in the same way, i.e. in 3 places - or at least for both Minitest and Test::Unit. Also explain why you've added allow_failures and fast_finish options in the commit note.

[junaruga]

@floehopper hi, ok let me try it. I missed test-unit case.

[floehopper]

@junaruga: Thanks. Merging.