Fix webpacker:compile task

Following rails/webpacker#2389
Requires webpacker v4.2+

Gemfile

@@ -4,7 +4,7 @@ source 'https://rubygems.org'
 
 gem 'rails', '~> 6.0.0'
 gem 'rails-i18n', '~> 6.0.0'
-gem 'webpacker', '~> 4.0'
+gem 'webpacker', '~> 4.2'
 
 # https://github.com/rails/rails/blob/v6.0.0.rc1/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb#L12
 gem 'sqlite3', '~> 1.3', '>= 1.3.6'

Rakefile

@@ -20,7 +20,7 @@ namespace :webpacker do
   task compile: %i[yarn_install load_app] do
     Dir.chdir(File.join(__dir__, 'spec/dummy')) do
       Webpacker.with_node_env('production') do
-        ensure_log_goes_to_stdout do
+        Webpacker.ensure_log_goes_to_stdout do
           exit! unless ::Webpacker.instance.commands.compile
         end
       end

heroku.gemfile

@@ -8,7 +8,7 @@ gem 'thredded', path: File.dirname(__FILE__)
 gem 'mustache'
 
 gem 'rails', '~> 6.0.0'
-gem 'webpacker', '~> 4.0'
+gem 'webpacker', '~> 4.2'
 
 # Make these gems available in production
 gem 'puma'

heroku.gemfile.lock

@@ -13,7 +13,7 @@ PATH
       kramdown (>= 2.0.0)
       kramdown-parser-gfm
       nokogiri
-      onebox (~> 1.9, >= 1.9.24)
+      onebox (~> 1.8, >= 1.8.99)
       pundit (>= 1.1.0)
       rails (>= 4.2.10, != 6.0.0.rc2)
       rb-gravatar
@@ -309,7 +309,7 @@ DEPENDENCIES
   tunemygc
   turbolinks
   twemoji
-  webpacker (~> 4.0)
+  webpacker (~> 4.2)
 
 RUBY VERSION
    ruby 2.6.5p114

spec/gemfiles/rails_6_0.gemfile

@@ -7,7 +7,7 @@ eval_gemfile '../../shared.gemfile'
 gem 'rails', '~> 6.0.0'
 gem 'rails-i18n', '~> 6.0.0'
 
-gem 'webpacker', '~> 4.0'
+gem 'webpacker', '~> 4.2'
 
 # https://github.com/rails/rails/blob/v6.0.0/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb#L13
 gem 'sqlite3', '~> 1.4'