ENH: Exit enstaller if running as root inside a venv

Running enstaller as root user once, changes the permissions for the
index files, and local dir, making it impossible to run enstaller
without root permissions in future. This commit prevents the user from
running enstaller as root inside a venv.
Merge pull request #81 from punchagan/fix-run-venv-sudo

ENH: Exit enstaller if running as root inside a venv