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)
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    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')