Add Ruby 2.4.0 to Travis CI.

[junaruga]

Add Ruby 2.4.0 to Travis CI.

[junaruga]

https://travis-ci.org/cucumber/cucumber-ruby/builds/197415458

Travis failed for only Ruby 2.4.0 test.
Because Travis rvm 2.4.0's installed simplecov-0.12.0 warns because of Ruby 2.4.0 Fixnum/Bignum warning feature.

expected `"/home/travis/.rvm/gems/ruby-2.4.0/gems/simplecov-0.12.0/lib/simplecov/configuration.rb:207: warning:...0/gems/simplecov-0.12.0/lib/simplecov/source_file.rb:30: warning: constant ::Fixnum is deprecated\n".empty?` to return true, got false (RSpec::Expectations::ExpectationNotMetError)

features/docs/cli/specifying_multiple_formatters.feature:23:in `Then the stderr should not contain anything'

[junaruga]

Latest version simplecov 0.13.0 is Ruby 2.4.0 compatibility.
simplecov-ruby/simplecov#544

[junaruga]

Possible solutions are

1. Update simplecov to latest version 0.13.0 by setting of .travis.yml before_install.

before_install:
  - gem update simplecov

2. Wait and use coveralls's next version release depending on simplecov 0.13.0.
   Same situation with Fix test suite for Ruby 2.4 compatibility. cucumber-ruby-core#120

[junaruga]

Close this PR because of #1077 .

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.