Enable the persistent object cache on a PHP 7 build on Travis CI

[johnbillion]

Currently there's an extra Travis CI build for PHP 5.6 which enables the persistent object cache using memcached.

The persistent object cache should be enabled on a PHP 7.3 build.

Once that's done, the 5.6 build with the persistent object cache can be removed in order to keep the build numbers down.

[netweb]

Testing 46633.diff​ in ​https://github.com/WordPress/wordpress-develop/pull/50

[netweb]

The CI Build: ​https://travis-ci.com/WordPress/wordpress-develop/builds/105618774

The CI job: ​https://travis-ci.com/WordPress/wordpress-develop/jobs/187367221

​https://travis-ci.com/WordPress/wordpress-develop/jobs/187367221#L414

$ sudo service memcached start
Starting memcached: memcached is already running.
memcached.

​https://travis-ci.com/WordPress/wordpress-develop/jobs/187367221#L491

$ if [[ "$WP_TRAVIS_OBJECT_CACHE" == "true" ]]; then
    cp tests/phpunit/includes/object-cache.php src/wp-content/object-cache.php
    echo "extension = memcached.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini
  fi

CI job passes, tests pass, no noticeable issues looking at the CI job log.

[SergeyBiryukov]

In 45027:

Build/Test Tools: Enable persistent object cache on a PHP 7.3 build instead of PHP 5.6.

Props netweb, johnbillion.
Fixes #46633.