Test load_train_file method in pylearn2.utils.serial

Fixes #517.

This adds a test that loads a YAML file both with and without environment variables being passed. The YAML file doesn't use an actual dataset so that the test will still run on Travis where the data files aren't available. It fails 74d395e as it should.

Also cleaned up the file while I was editing it anyway and removed it from the whitelist.

@bouthilx