TST: fix cumcount test on 32-bit env
Fix `test_groupby.test_cumcount_empty()` on 32-bit platforms by specifying expected dtype of 'int64'.

```
======================================================================
FAIL: test_cumcount_empty (pandas.tests.test_groupby.TestGroupBy)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/gmd/github_shadow/pandas/pandas/tests/test_groupby.py", line 2579, in test_cumcount_empty
    assert_series_equal(e, ge.cumcount())
  File "/home/gmd/github_shadow/pandas/pandas/util/testing.py", line 423, in assert_series_equal
    assert_attr_equal('dtype', left, right)
  File "/home/gmd/github_shadow/pandas/pandas/util/testing.py", line 407, in assert_attr_equal
    assert_equal(left_attr,right_attr,"attr is not equal [{0}]" .format(attr))
  File "/home/gmd/github_shadow/pandas/pandas/util/testing.py", line 390, in assert_equal
    assert a == b, "%s: %r != %r" % (msg.format(a,b), a, b)
AssertionError: attr is not equal [dtype]: dtype('int32') != dtype('int64')
```

We should add a note about using int64 instead of int. Not that it will
necessarily resolve this completely.
 Thanks for catching this! I'm hoping we can get an automatic 32bit build from Travis at some point. 
 Thanks Jeff..  For completeness, here's the open issue tracking 32-bit env support in Travis: https://github.com/travis-ci/travis-ci/issues/986
