Removed an excessive argument from a call to check_range

This fix removes the following exception:

Traceback (most recent call last):
  File "./validate-yaml.py", line 55, in <module>
    main()
  File "./validate-yaml.py", line 51, in main
    process(schema_file, yaml_file)
  File "<string>", line 2, in process
  File "/usr/lib/python2.7/site-packages/contracts/main.py", line 269, in contracts_checker
    result = function_(*args, **kwargs)
  File "./validate-yaml.py", line 29, in process
    schema.check(data)
  File "/usr/lib/python2.7/site-packages/Rx/__init__.py", line 303, in check
    self.value_schema.check(v, stack)
  File "/usr/lib/python2.7/site-packages/Rx/__init__.py", line 403, in check
    self.optional[field].check( value[field], stack )
  File "/usr/lib/python2.7/site-packages/Rx/__init__.py", line 209, in check
    if self.length and not self.length(len(value), stack):
TypeError: check_range() takes exactly 1 argument (2 given)