TestTube version attribute does not perform as documented
🐛 Bug
The docs for TestTube logger state:

If version is not specified the logger inspects the save for existing versions, then automatically assigns the next available version.

However, this does not happen.
To Reproduce
>>> from pytorch_lightning.loggers import TestTubeLogger
>>> logger = TestTubeLogger('tb_logs', name='my_model')
>>> logger.version
>>> assert logger.version is None

Expected behavior
I would expect the TestTube logger performs something similar to:
https://github.com/PyTorchLightning/pytorch-lightning/blob/master/pytorch_lightning/loggers/tensorboard.py#L177