Update Travis configuration file to include tests for WP 5.5

[imath]

WordPress has bumped their trunk to 5.6 and created a 5.5 branch.

[imath]

In 12705:

Build Tools: update Travis config to include tests for WP 5.5

Also, update the Node version used for Travis jobs

PS: Let’s welcome WordPress ”Eckstine” <3

Fixes #8346

[imath]

7.2 jobs are failing since this commit. Looks like v12 of Node is problematic on precise dist. Let's try to fix this.

[imath]

In 12706:

Build Tools: Use travis‘ trusty dist for PHP 7.2 jobs

To avoid the error about the fact the installed by default PHPunit version on trusty does not support PHP 7.2, only run the phpunit --version command if current job is not concerning PHP 7.2.

See #8346