Remove reference to CSV class

This moves any uninitialized constant error to the calling code, which hopefully makes it clear to devs that they should` require "csv"` before using the CsvEnumerator.

lib/job-iteration/csv_enumerator.rb

@@ -20,7 +20,7 @@ class CsvEnumerator
     #   csv = CSV.open('tmp/files', { converters: :integer, headers: true })
     #   JobIteration::CsvEnumerator.new(csv).rows(cursor: cursor)
     def initialize(csv)
-      unless csv.instance_of?(CSV)
+      unless csv.class.name == "CSV"
         raise ArgumentError, "CsvEnumerator.new takes CSV object"
       end