Allow updating the value of an enum column

Something along the lines of 

``` ruby
class Table
  ... 
  def change(old_value, new_value)
    @connection.execute "UPDATE #{@connection.quote_table_name @name}
    SET value = #{@connection.quote old_value} WHERE value = #{@connection.quote new_value}"
  end
end
```

in https://github.com/howaboutwe/enum_table/blob/master/lib/enum_table/schema_statements.rb#L66
