2: add virtualenv

Tested with:
```
proppy@noir:~/python/2$ docker run -ti python:2 bash
root@6b01e15ce5d6:/usr/src/python# virtualenv
```

:3 already has `pyvenv`

Why not do `virtualenv` in a separate `RUN` line, and thus have something that's easy to copy/paste into `3/Dockerfile` too? :smile: