spree:install task have been renamed to solidus:install

Solidus reference commit:
solidusio/solidus#3538

Author: Mattia Roccoberton

lib/solidus_dev_support/templates/extension/bin/sandbox.tt

@@ -67,7 +67,7 @@ unbundled bundle install --gemfile Gemfile
 
 unbundled bundle exec rake db:drop db:create
 
-unbundled bundle exec rails generate spree:install \
+unbundled bundle exec rails generate solidus:install \
   --auto-accept \
   --user_class=Spree::User \
   --enforce_available_locales=true \