Centralize Travis scripts

[PeterBowman]

Issue #48 led to the creation of a scripts/travis/ directory in several (perhaps all) Travis CI-enabled repositories: example. This could be avoided thanks to the new Build Configuration Imports beta feature.

Build Config Imports allow you to create modular build configuration systems that can be shared across many repositories, and not only spare the work of manually duplicating build configuration across those repositories, but also standardize how code is built and deployed in your organization.

[PeterBowman]

Provided GH Actions can handle CI (#87 (comment)), this issue will become a WONTFIX.

[PeterBowman]

As a side note, Ubuntu Focal (20.04) images are now available in Travis CI: https://blog.travis-ci.com/2020-08-10-focal-build-environment.

[PeterBowman]

Provided GH Actions can handle CI (#87 (comment)), this issue will become a WONTFIX.

YARP will migrate to GH Actions because of https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing, let's watch robotology/yarp#2410 closely.

[PeterBowman]

Closing as WONTFIX because of the ongoing migration to GitHub Actions: #91. Note we don't need the GHA equivalent of those three shell scripts whatsoever: repo checkout, build caching and SSH configuration is entirely taken care of by specific actions, and coverage support will be discontinued for now (future: #71?).