Moving solidus_sample to be development dependency

Solidus Sample gem includes seed data to setup local development in
an easy way, it includes catalogs for countries, states, products,
taxonomies/taxons and images. This gem itself needs about 10mb of disk
space and most of the times it is only needed in local development
or one time.

README.md

@@ -150,6 +150,9 @@ bin/rails db:seed
 bin/rails spree_sample:load
 ```
 
+`solidus_sample` is a development dependency, if you need to run `spree_sample:load`
+in production environments, then you have to add it to your local Gemfile `gem 'solidus_sample'`.
+
 There are also options and rake tasks provided by
 [solidus\_auth\_devise](https://github.com/solidusio/solidus_auth_devise).
 

solidus.gemspec

@@ -24,5 +24,6 @@ Gem::Specification.new do |s|
   s.add_dependency 'solidus_api', s.version
   s.add_dependency 'solidus_backend', s.version
   s.add_dependency 'solidus_core', s.version
-  s.add_dependency 'solidus_sample', s.version
+
+  s.add_development_dependency 'solidus_sample', s.version
 end