Fixed logging of serf agent messages when /bin/sh is not a symlinked bash.

When running on a system where `/bin/sh` is not a symlink to `/bin/bash` (eg. Ubuntu), messages from the serf agent are displayed on the terminal rather than in `$SERF_AGENT_LOG` as expected. I traced the bug to the use of `&>>` in the service script, which according to "man bash" is shortcut to the more compatible `>>$SERF_AGENT_LOG 2>&1`.