Fix deprecation warning when used with Rails 4.1

Rails 4.1 deprecates ActiveRecord::Migrator.proper_table_name in favor of ActiveRecord::Migration#proper_table_name at https://github.com/rails/rails/commit/8c5d62f79669fcf67d106036177d731703f3aa44
Move proper table name to its own method
Merge pull request #125 from ffmike/patch-1

Fix deprecation warning when used with Rails 4.1