Bundle solidus_admin >= 0.2 in Solidus installer

Currently, running `rails g solidus:install` on a new rails app
bundles `solidus_admin` with version `0.0.0`, which is empty.

This leads to inconsistencies, the first one being the dedicated
`solidus_admin` generator is not found and consequently cannot be
run.

core/lib/generators/solidus/install/install_generator.rb

@@ -179,7 +179,7 @@ def install_solidus_admin
  return unless options[:admin_preview]
 
  say_status :installing, "SolidusAdmin", :blue
- bundle_command 'add solidus_admin'
+ bundle_command 'add solidus_admin -v ">= 0.2"'
  generate 'solidus_admin:install'
  end