Bump ransack dependency up to 2.0

Ransack 2.0.1 is the only version compatible with rails 5.2.1.

Rails < 5.0 is no more supported since it needs to use ransack 1.8.
This is not an issue since we are not supporting rails < 5.1 anyway.

core/solidus_core.gemspec

@@ -38,6 +38,6 @@ Gem::Specification.new do |s|
   s.add_dependency 'monetize', '~> 1.1'
   s.add_dependency 'paperclip', ['>= 4.2', '< 6']
   s.add_dependency 'paranoia', '~> 2.4'
-  s.add_dependency 'ransack', '~> 1.8'
+  s.add_dependency 'ransack', '~> 2.0'
   s.add_dependency 'state_machines-activerecord', '~> 0.4'
 end