Travis CI: dist: xenial because Trusty is EOL (#621)
Travis CI: dist: xenial because Trusty is EOL (#621)

Travis CI: Try other distros because Trusty is EOL

Dist: xenial

* `dist: bionic` fails on `Package 'libstdc++-4.9-dev' has no installation candidate`
* `dist: focal` fails because you cannot install legacy Python on it.

https://github.com/zmap/zmap/pull/621

Co-authored-by: David Adrian <bc848bfde29d51c62549d82ec381a93d82896f16@gmail.com>
zmap/zmap,zmap/zmap,zmap/zmap