Add a post install/update message

We already introduced this post-install message in 2.11.5
when we started requesting users to run a rake task after
bumping Solidus.

Even if we don't have any specific rake task here in 3.0 we
can keep this convention so we always have a place where to
put this kind of tasks.

Also, we can be consistent between versions in terms of how
we request people to upgrade Solidus:

1. Change Gemfile
2. Run `bin/rails solidus:upgrade`

Even if it does nothing special, I think it's a positive thing
to have.

core/lib/tasks/upgrade.rake

@@ -2,12 +2,14 @@
 
 namespace :solidus do
   namespace :upgrade do
-    desc "Upgrade Solidus to version 2.11"
-    task two_point_eleven: [
-        'solidus:migrations:migrate_default_billing_addresses_to_address_book:up',
-        'solidus:migrations:migrate_address_names:up'
+    task three_point_zero: [
+        'railties:install:migrations',
+        'db:migrate'
       ] do
-      puts "Your Solidus install is ready for Solidus 2.11"
+      puts "Your Solidus install is ready for Solidus 3.0"
     end
   end
+
+  desc "Upgrade to the current Solidus version"
+  task upgrade: 'upgrade:three_point_zero'
 end

core/solidus_core.gemspec

@@ -42,4 +42,24 @@ Gem::Specification.new do |s|
   s.add_dependency 'kt-paperclip', '~> 6.3'
   s.add_dependency 'ransack', '~> 2.0'
   s.add_dependency 'state_machines-activerecord', '~> 0.6'
+
+  s.post_install_message = <<-MSG
+-------------------------------------------------------------
+                Thank you for using Solidus
+-------------------------------------------------------------
+If this is a fresh install, don't forget to run the Solidus
+installer with the following command:
+
+$ bin/rails g solidus:install
+
+If you are updating Solidus from an older version, please run
+the following commands to complete the update:
+
+$ bin/rails solidus:upgrade
+
+Please report any issues at:
+- https://github.com/solidusio/solidus/issues
+- http://slack.solidus.io/
+-------------------------------------------------------------
+MSG
 end