Remove Ruby 2.4 from allow_failures in .travis.yml.
Remove Ruby 2.4 from allow_failures in .travis.yml.

Because coveralls 0.8.20 for Ruby 2.4 compatibility was released.
See also https://github.com/lemurheavy/coveralls-ruby/pull/117

cucumber/cucumber-ruby-core,kamenlitchev/cucumber-ruby-core