TST: fix failing #2752 tests for 32-bit builds

[stephenwlin]

fix failing 32-bit travis builds after #2708, #2752

still unclear whether the following needs to be fixed on 32-bit:

In [29]: p.DataFrame([1,2]).dtypes
Out[29]: 
0    int32
Dtype: object

In [30]: p.DataFrame({'a': [1,2]}).dtypes
Out[30]: 
a    int64
Dtype: object

so leaving the relevant test hardcoded to int64 for now