pip module - expand user home directory variables in virtualenv parameter

Just a one-liner to expand home directories - I'm installing a service with a virtualenv in the home directory of the user that runs it, something like:
```yaml
- name: install django, for example
  pip: name=django
       version=1.5
       virtualenv=~myservice/env
```
Without this patch, the task is looking for the virtualenv in ```/home/ansible-ssh-user/~myservice/env``` without the intended expansion.