Fix tmux guard
Merge branch 'fix-tmux-support' into develop

Ref #672
Use “tmux refresh -S” to check for tmux presence

Reason: tmux may be launched and $TMUX set, but current shell still not under 
tmux (and most likely there are no active session). In this case `tmux refresh 
-S` may fail with “no clients” message, which is annoying.
Merge branch 'fix-no-clients-error' into develop