Change "source" to "." in autojump.sh

[abravalheri]

Hi @wting thank you very much for autojump! Amazingly useful.

This PR makes the autojump.sh file more POSIX-compliant, which is useful in some OSs that link /bin/sh to secondary shells for performance improvements while still using bash|zsh|... as the default shell

References:

• https://wiki.ubuntu.com/DashAsBinSh
• padde/jump.vim#3