Petitboot v1.9.1 released

[Samuel Mendoza-Jonas]

Petitboot v1.9.1 has been tagged, including:

- Fixes around network requery corner cases, where the DHCPv6 client
wouldn't be exited and autoboot could be accidentally cancelled.

- Extending the 'boot' pb-event to accept a name parameter.

- 'Lockdown' mode in the ncurses menu fixed to not exit to shell.

- The kboot parser extended to recognise the 'default' parameter.

- Updates to the travis & docker scripts.

- An update to the README file