Don't suggest that they add to their `.profile` anymore because:
1. They would have to restart the desktop session for their change to be
reflected, or source the file manually;
2. An error in `.profile` may prevent logging in;
3. The `rbenv()` shell function and shell completions won't be available
in Terminal since it doesn't start bash in login mode by default.
Therefore, suggest that they use `.bashrc` instead. This will be
immediately reflected in a new Terminal tab. If bash is started in login
mode somehow, the default `.profile` is set up to source `.bashrc` anyway.
Also, don't suggest restarting the shell environment with
`exec $SHELL -l`, since we don't know what was the original mode that
their shell was started in. (OS X Terminal.app will be login mode,
Ubuntu Terminal has non-login mode by default.) Mode can be checked with:
* bash: `shopt -q login_shell`
* zsh: `[[ $options[login] = "on" ]]`
But since this is gnarly, let's just avoid it altogether and go the
easy route.
Closes#305, fixes#373, reverts #286