Modify a gemfile in Appraisals

`appraise 'rails-5_no_sprockets_webpacker_3'` intends to test webpacker v3.
Now, webpacker v4.0.0.pre has been released, so we need to modify this file.

Author: ttanimichi

Appraisals

@@ -14,7 +14,7 @@ end
 #   load in certain tests or not.
 appraise 'rails-5_no_sprockets_webpacker_3' do
   gem 'rails', '~> 5.0.0'
-  gem 'webpacker', '>= 3.0'
+  gem 'webpacker', '~> 3.0'
   # This ExecJS backend provides stateful context
   # which the default nodejs backend does not
   gem 'mini_racer', :platforms => :mri

gemfiles/rails_5_no_sprockets_webpacker_3.gemfile

@@ -3,7 +3,7 @@
 source "http://rubygems.org"
 
 gem "rails", "~> 5.0.0"
-gem "webpacker", ">= 3.0"
+gem "webpacker", "~> 3.0"
 gem "mini_racer", platforms: :mri
 gem "therubyrhino", platforms: :jruby