[Fix #541] Do not auto-load screen/tmux inside of Emacs/Vim

When Emacs and Vim are launched from outside of an interactive shell,
$TMUX and $STY are not set; check for Emacs and Vim environment
variables instead.

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Fix a test operator bug introduced in e5a0e33
[Fix #551] Rename tmux auto-start session
Inform the user to load ruby as late as possible when using RVM
Merge branch 'master' of https://github.com/sorin-ionescu/prezto