Add ruby-head to Travis.

[junaruga]

I want to add Ruby 2.4.0 and ruby-head to Travis.
The motivation and the detail is same with cucumber/cucumber-ruby#1087

Thanks.

[junaruga]

Error on Travis.
https://travis-ci.org/cucumber/cucumber-ruby-wire/builds/209719754

Updating git://github.com/cucumber/cucumber-ruby.git
Updating git://github.com/cucumber/cucumber-ruby-core.git
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
Could not find gem 'cucumber-core (~> 1.3.0) ruby', which is required by gem
'cucumber (>= 0) ruby', in any of the sources.

[junaruga]

After my additional change, the bundle install was success.

The remove-wire-protocol-to-plugin and master branch looks same actually.

$ git clone [email protected]:cucumber/cucumber-ruby.git
$ cd cucumber-ruby
$ diff remove-wire-protocol-to-plugin master
  => No difference

But error on Ruby 1.9.3 and Jruby.

Those have below some error.

/home/travis/.rvm/gems/ruby-1.9.3-p551/bundler/gems/cucumber-ruby-057c3dc1bf20/lib/cucumber/rb_support/rb_language.rb:3:in `require': /home/travis/.rvm/gems/ruby-1.9.3-p551/bundler/gems/cucumber-ruby-057c3dc1bf20/lib/cucumber/rb_support/rb_dsl.rb:56: syntax error, unexpected tPOW (SyntaxError)

      def World(*world_modules, **namespaced_world_modules, &proc)