Suggest a virtual environment.

[neilteng]

I suggest a virtual environment for this tutorial

For one thing, it decouples the change in new relase of tf and the development environment we use. it saves authors' effort to answer tf version related problem and delegate them back to tf developers.
And it also saves readers effort to figure out missing package. e.g. When I run the 5-1, it tells me I miss the package pillow which is not explicitly imported.

Best
Neil