Lock Rails version up to 5.2.1

There are incompatibilities between core code and Rails 5.2.2
that we need to investigate better.

Locking Rails to 5.2.1 should allow us to have all the ecosystem
working in the meantime, for example this should immediately fix:

- extensions tests against master
- sandbox
- heroku demo

ref solidusio#2989

core/solidus_core.gemspec

@@ -24,7 +24,7 @@ Gem::Specification.new do |s|
     actionmailer actionpack actionview activejob activemodel activerecord
     activesupport railties
   ].each do |rails_dep|
-    s.add_dependency rails_dep, ['>= 5.1', '< 5.3.x']
+    s.add_dependency rails_dep, ['>= 5.1', '< 5.2.2']
   end
 
   s.add_dependency 'activemerchant', '~> 1.66'