Just trying to keep the README.md up to date with Ruby versions

Installation Step 1 is a one liner

Changing Ruby versions references to latest versions to keep somewhat up-to-date.

Not a big change, just thought it's always good to make it easier for a beginner

remove "+=" operator in rbenv-rehash to support bash-3.0

help for versions and commands, including their options

Fix typos in documentation

Prevent $command from leaking outside of function

Add /usr/lib/rbenv/hooks to hook search path

This will help with the packaging of rbenv plugin that contain hooks in
Debian.

rbenv-init arguments should be allowed in any order

Conflicts:
	libexec/rbenv-rehash

Made ksh portability changes

Only run compctl if it's an interactive shell

Added specific message for ksh in identifying the proper shell
initialization file.

Changed rbenv functiond definition to be more portable.
Shell functions should be defined by using the function command or
using the parenthesis grammar, but using both is not portable:

rbenv() {...  -or-
function rbenv { ...

Based on the approach by @imajes

Quoting the argument to greadlink to make it work with paths containing spaces

$OLDPWD is a standard shell variable that contains the previous working
directory as set by the "cd" command. No need to save $PWD to some
custom variable.

(We could also have used "cd -" but it prints out $OLDPWD too.)

A trap on the special signal EXIT is executed before the shell
terminates. EXIT actually covers SIGINT and SIGTERM as well, and
we don't need any extra traps for them.

See bash(1) and "help trap" in bash.

Prints $RBENV_ROOT (defaults to ~/.rbenv)

Otherwise, `RBENV_DIR=bin rbenv version-file` loops indefinitely

Fix command to modify ~/.bash_profile in user's home directory