Bump mosquitto versions to latest available

[leojrfs]

Hey!

Small PR just to bump the mosquitto versions in the docker images to match the latest mosquitto releases:

• 1.6.14 to 1.6.15
• 2.0.15 to 2.0.18

cheers

[iegomez]

Fixed in https://github.com/iegomez/mosquitto-go-auth/pull/317