moved _powerline_prompt to front of $PROMPT_COMMAND in bash binding for ...

This change fixes errant behavior of the last_status segment in bash when $PROMPT_COMMAND already exists before the powerline.sh bash binding is sourced. On OS X for instance, $PROMPT_COMMAND is set to `update_terminal_cwd` by default.

Without this, last_status just gets the return code of the previous items in $PROMPT_COMMAND.