Fix OverflowError from tests on 32bit systems.

This fixes the following error that occurs while executing the tests on a 32-bit system:
```python
======================================================================
ERROR: test_core.test_interpose
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/erikw/git/toolz/toolz/itertoolz/tests/test_core.py", line 170, in test_interpose
    assert "a" == first(rest(interpose("a", range(10000000000))))
OverflowError: Python int too large to convert to C long
```