Modifications to the tests in 'pylearn2/tests/rbm/test_ais.py' in order to reduce their running time.

Running the command written below previously took longer than 4 hours, so I modified some parameters in order to reduce the running time to 20 minutes.
```
THEANO_FLAGS=mode=DEBUG_MODE nosetests test_ais.py
```
This is not a good idea. It basically makes the default configuration worthless.

In truth this section of code should come with a comment saying that this is approximately what's needed for a 500 hidden unit RBM.

For the test, you should pass in the smaller hstack with the `betas=` parameter instead of changing `dflt_beta`.