Fix missing class (why do I need this??)

Author: eapache

lib/graphql/schema/scalar.rb

@@ -49,6 +49,7 @@ def validate_non_null_input(value, ctx)
             err
           end
 
+          require 'graphql/coercion_error'
           if coerced_result.nil?
             str_value = if value == Float::INFINITY
               ""