Update .travis.yml

[KapitanOczywisty]

Fix for recent build failures

[KapitanOczywisty]

@sadick254 can you merge this?

Edit: trusty was queued very quickly contrary to xenial, maybe better to update gcc/g++ and keep trusty?

[sadick254]

@KapitanOczywisty we could update gcc/g++ on xenial. The queuing is something external and could change, we should not base our solutions on external factors we have no control over. I am happy with this change. Thank you @KapitanOczywisty .