Solidus gem: Require solidus_legacy_promotions

Solidus main already depends on `solidus_legacy_promotions`, but we
forgot to require it in the original PR. This makes sure people still
have the legacy promotion system when they upgrade to the next Solidus
version.

Author: mamhoff

lib/solidus.rb

@@ -4,3 +4,4 @@
 require 'solidus_api'
 require 'solidus_backend'
 require 'solidus_sample'
+require 'solidus_legacy_promotions'