Update plex-install.sh

[kabroxiko]

Description

Add gpg dependency which is needed to avoid warning when using plexupdate.sh

You can currently reproduce this issue by:

• Executing the installation script inside de LXC console
• Running update manually:

/opt/plexupdate/plexupdate.sh -f

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)