releasetools: Fix the doctest for rangelib.py.

$ python rangelib.py
**********************************************************************
File "rangelib.py", line 323, in __main__.RangeSet.next_item
Failed example:
    list(rangelib.RangeSet("10-19 3 5 7").next_item())
Exception raised:
    Traceback (most recent call last):
      File "/usr/lib/python2.7/doctest.py", line 1315, in __run
        compileflags, 1) in test.globs
      File "<doctest __main__.RangeSet.next_item[2]>", line 1, in <module>
        list(rangelib.RangeSet("10-19 3 5 7").next_item())
    NameError: name 'rangelib' is not defined
**********************************************************************
1 items had failures:
   1 of   3 in __main__.RangeSet.next_item
***Test Failed*** 1 failures.

Test: python rangelib.py
Change-Id: I6c791ff4453f40d90b34f97b881460da9878fe64