updated autojump.bash, removing export

exporting the prompt seems to be too harsh and breaks the shell in Emacs as the PROMPT_COMMAND for xterm (say) overwrites the 'dumb' PROMPT_COMMAND, which now is 'contaminated' with ASCII escape strings.
Merge pull request #198 from pank/master

Disable export of PROMPT_COMMAND for bash shells.

Not using export means sub shells can't see the env
variable changes, but this shouldn't be an issue since
new sub shells should be sourcing ~/.bashrc anyway.