Update powerline for new psutil

Fixes #835
Switch comments
Fix tests for new psutil
Merge pull request #838 from ZyX-I/new-psutil-fixes

New psutil fixes
Put _powerline_prompt at the start

Reason: $? may be overridden by the previous commands in PROMPT_COMMAND that do 
not keep it like _powerline_prompt does (see `return $last_exit_code`). 
(@ascrane)

Closes #836
Merge branch 'bash-keep-status' into develop