Adding apt-get update as suggested by manual
Adding apt-get update as suggested by manual

Manual at: https://docs.travis-ci.com/user/installing-dependencies/

Signed-off-by: Simon Egli <e67c3bf81b20f6ecbfcabd4082caedf40229f01c@bbv.ch>

DoZh/Serial-transparency-over-MQTT,DoZh/Serial-transparency-over-MQTT