.github/workflows/travis-ci.yml: unix-nanbox, bare-arm: Use ubuntu-18.04.
.github/workflows/travis-ci.yml: unix-nanbox, bare-arm: Use ubuntu-18.04.

Package installation issue currently, and 16.04 is EOL next month.

Change-Id: I482b266a64147b645bc9ffe719d5c2f389931885
Signed-off-by: Paul Sokolovsky <9dc5061f178bb11813e14f1ed02ba27eaf21fa5d@users.sourceforge.net>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython