Use $WORKON_HOME consistently for virtualenv.

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.
Merge pull request #377 from edx/jtauber/fix-virtualenv-directory

Use $WORKON_HOME consistently for virtualenv.