tmux: Spawn non-login shells

tmux spawns login shells by default. When Bash starts as a login shell,
it runs the '/etc/profile' initialization file. On Darwin, this results
in 'path_helper' to to be executed. 'path_helper', in turn, prepends
system directories, such as '/usr/bin', to $PATH.

If tmux is started from a login shell, $PATH is already configured when
'path_helper' runs. While '.bash_profile' (and consequently '.bashrc')
runs after 'path_helper' and reconfigures $PATH, rbenv does not prepend
a second instance of its shim directory to $PATH. As a result, 'ruby'
runs the system Ruby instead of rbenv's shim.

Fix this by configuring tmux to spawn non-login shells.