reduce memory requirements to run tests

You can check the memory usage with the following command:

```
/usr/bin/time ./setup.py nosetests
[...]
OK
16.08user 1.20system 0:18.00elapsed 96%CPU (0avgtext+0avgdata 105740maxresident)k
0inputs+810400outputs (0major+151138minor)pagefaults 0swaps
```
The number in question is "105740maxresident" (kilobytes) = 103.2 megabytes.