Just to confirm, you are trying to run the lesson1.ipynb (cats v dogs) for the 2017/18 version (v2) of the course? Note that the 2018/19 version (v3) should have their videos available online soon, so it might be better / easier to wait for that, and try to use fastai library v1 for it.
If you do mean to run the 2017/18 course, then as @RogerS49 mentioned, you need a different version of fastai, namely the older (pre-release) v0.7, with all its older dependencies including older pytorch torchvision etc. I am not familiar with Google Colab, but you should probably refer to this thread for fastai v0.7 setup.