Drop support for Rails 4.2

[jcoyne]

Rails 4.2 is no longer supported by the Rails core team and the current build gives problems like this:

An error occurred while loading ./spec/config_env_spec.rb.

Failure/Error: Rails.application.initialize!

Sprockets::Railtie::ManifestNeededError:

  Expected to find a manifest file in `app/assets/config/manifest.js`

  But did not, please create this file and use it to link any assets that need

  to be rendered by your app:

  Example:

    //= link_tree ../images

    //= link_directory ../javascripts .js

    //= link_directory ../stylesheets .css

  and restart your server

# ./gemfiles/vendor/bundle/ruby/2.6.0/gems/sprockets-rails-3.2.1/lib/sprockets/railtie.rb:105:in `block in <class:Railtie>'

# ./gemfiles/vendor/bundle/ruby/2.6.0/gems/railties-4.2.11/lib/rails/initializable.rb:30:in `instance_exec'

# ./gemfiles/vendor/bundle/ruby/2.6.0/gems/railties-4.2.11/lib/rails/initializable.rb:30:in `run'

# ./gemfiles/vendor/bundle/ruby/2.6.0/gems/railties-4.2.11/lib/rails/initializable.rb:55:in `block in run_initializers'

# ./gemfiles/vendor/bundle/ruby/2.6.0/gems/railties-4.2.11/lib/rails/initializable.rb:54:in `run_initializers'

# ./gemfiles/vendor/bundle/ruby/2.6.0/gems/railties-4.2.11/lib/rails/application.rb:352:in `initialize!'

# ./spec/app/rails_4.2/config/environment.rb:5:in `<top (required)>'

# ./spec/spec_helper.rb:37:in `require'

# ./spec/spec_helper.rb:37:in `<top (required)>'

# ./spec/config_env_spec.rb:1:in `require'

# ./spec/config_env_spec.rb:1:in `<top (required)>'

TRAVIS_COMPILER 

TRAVIS_RUBY_VERSION 2.6.0

ENV

[jcoyne]

Duplicate of #251