Create /usr/venv symlink.
Create /usr/venv symlink.

* This is a temporary hack to create a path that
  pip v.1.4 mistakenly expects to exist.
* Once https://github.com/pypa/pip/issues/1074 is fixed,
  then we can remove the symlink.

Fixes (for now) #8.

toffer/minecloud-ami,toffer/minecloud-ami