Replace POWERLINE_OLD_PROMPT_COMMAND with the code used by [z][1]

Does not fix anything, but is three lines less and is probably the way other 
tools expect us to do.

[1]: https://github.com/rupa/z
Replace `jobs` with `jobs -p`


If some long shell code is run in the background it may be shown as more then 
one job:

    bash-4.2$ {
    > echo abc | while read line ; do
    > sleep 1s
    > sleep 1s
    > done
    > } &
    [1] 9401
    bash-4.2$ jobs
    [1]+  Running                 { echo abc | while read line; do
        sleep 1s; sleep 1s;
    done; } &

`jobs -p` print exactly one job in this case.
Document problem found when exploring #749
Fix functional shell tests in travis

Hostname in travis contains random numbers meaning that it occasionally may 
contain a PID as well. Thus it must be replaced first.
Merge pull request #754 from ZyX-I/improve-bash-bindings

Improve bash bindings