Prevent tmuxinator from sharing rbenv environment

When the `mux` command runs under rbenv the RBENV_VERSION and RBENV_DIR
environment variables are set. They're inherited in the tmuxinator
process that `mux` launches, so we need to clobber them ourselves.

See https://github.com/aziz/tmuxinator/issues/99
Don't append ~/.rbenv/bin to PATH

It confuses things, preventing the rbenv shim for gem being found first
in PATH.