Add Solidus Auth Devise to Sandbox

Without adding the gem, the `rake solidus:auth:install` command further
down the script will fail.

Author: mamhoff

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

@@ -47,6 +47,7 @@ cat <<RUBY >> Gemfile
 gem 'solidus', github: 'solidusio/solidus', branch: '$SOLIDUS_BRANCH'
 gem 'rails-i18n'
 gem 'solidus_i18n'
+gem 'solidus_auth_devise'
 
 gem '$extension_name', path: '..'