Use the latest Rubies on Travis

[koic]

No description provided.

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[koic]

Travis CI has failed with the following matrix.

• Ruby 2.4 / Rails 4.0 (Arel 4.0)
• Ruby 2.4 / Rails 4.1 (Arel 5.0)

TypeError (Cannot visit Integer):
  /home/travis/build/rails/activerecord-session_store/gemfiles/vendor/bundle/ruby/2.4.0/gems/arel-5.0.1.20140414130214/lib/arel/visitors/visitor.rb:28:in `rescue in visit'
  /home/travis/build/rails/activerecord-session_store/gemfiles/vendor/bundle/ruby/2.4.0/gems/arel-5.0.1.20140414130214/lib/arel/visitors/visitor.rb:22:in `visit'
  /home/travis/build/rails/activerecord-session_store/gemfiles/vendor/bundle/ruby/2.4.0/gems/arel-5.0.1.20140414130214/lib/arel/visitors/to_sql.rb:330:in `visit_Arel_Nodes_Limit'
https://travis-ci.org/rails/activerecord-session_store/builds/211658656

https://travis-ci.org/rails/activerecord-session_store/builds/211658656

The reason I think about is that the following PR is not backported to Arel 4.0 and Arel 5.0.

rails/arel#437

But I think these backports are not necessary. Because, as far as I know, Rails 4.0 and Rails 4.1 are not already supported.

Therefore, I propose to dropped support for Rails 4.0 and Rails 4.1 in this travis.yml.

What do you think about this?