Add a post deploy hook to install Procfile

- Virtual environment will be linked in to the current directory in
  'venv'
- Procfile now gets created by the post deploy hook which is called by
  capistrano
Remove log directory

This is handled by capistrano at deployment now. The log directory is
created in shared and symlinked into release.
Merge pull request #33 from alphagov/capistrano-deployment

Add a post deploy hook to install Procfile