Issue downloading OpenFire

[Mateo]

🐛 Bug

This test do not pass anymore :

We have a problem with OpenFire, the links to the images work less and less.

Now this test doesn't work anymore:

pyro-vision/test/test_datasets.py

Line 103 in db08df7

 self.assertAlmostEqual(len(train_set) + len(test_set), len(extract), delta=20) 

In the past we have already increased the threshold from 15 to 20 to get around this problem.

I propose to do it again with a big margin, let's take 30.

On the other hand we have to absolutely store this dataset somewhere soon and change the download mode.

[F-G Fernandez]

Closed by #153