Some parts of the `create-dev-env/sh` script use `$WORKON_HOME` and
others hardcode `$HOME/.virtualenvs` which means if you **don't** put
your virtualenvs under `$HOME/.virtualenvs`, the script doesn't work at
all.
This commit removes the hardcoding of virtualenv location and
consistently uses `$WORKON_HOME` everywhere.
047da7a7ce