add a version check

Author: atomical

lib/schema_plus/core/schema_dump.rb

@@ -105,10 +105,12 @@ def assemble(stream, typelen, namelen)
               pr += ',' unless options.blank?
               stream.write "%-#{namelen+3}s " % pr
             end
-            if options[:default].is_a?(Proc)
-              default = options.delete(:default)
-              stream.write "default: -> { \"#{default.call}\" }"
-              stream.write ", " unless options.blank?
+            if Gem::Version.new(ActiveRecord::VERSION::STRING) >= Gem::Version.new('5.0.0')
+              if options[:default].is_a?(Proc) && 
+                default = options.delete(:default)
+                stream.write "default: -> { \"#{default.call}\" }"
+                stream.write ", " unless options.blank?
+              end
             end
             stream.write options.to_s.sub(/^{(.*)}$/, '\1') unless options.blank?
             stream.write ' ' unless options.blank? or comments.blank?