Cleanup Travis config

[jeroen]

Using declarative installation of apt packages will result in better caching. Also sudo is deprecated and dist: trusty is EOL soon, it's better to use the default linux distribution.

[yutannihilation]

It seems Travis still uses Trusty here. So, we might need some additional tweaks when actually migrate to Xenial.

https://travis-ci.org/tidyverse/ggplot2/jobs/541740990#L9

Anyway, I think we should merge this as Trusty is already EOL. Unless there's some concern, I'll merge this.

c.f. https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment

[yutannihilation]

Confirmed tests run fine on Xenial with my forked repo: https://travis-ci.org/yutannihilation/ggplot2/builds/563266792

The main repo stays on Trusty even after I cleared the build cache, but I think we can just wait for their migration is done: https://travis-ci.org/yutannihilation/ggplot2/builds/563266792

[lock]

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