[Travis] Fixed REST tests failing for PHP < 7.3

[mnocon]

Question	Answer
JIRA issue	https://jira.ez.no/browse/EZP-31268
Bug/Improvement	issue in tests
New feature	no
Target version	6.13, 7.5 (not needed for master, as it requires PHP 7.3)
BC breaks	no
Tests pass	should
Doc needed	no

The REST functional tests job in ezplatform-http-cache is failing: https://travis-ci.org/ezsystems/ezplatform-http-cache/jobs/626775072

This is caused by the usage of heredoc syntax that has been allowed only since PHP 7.3:
https://www.php.net/manual/en/migration73.new-features.php#migration73.new-features.core.heredoc

This PR:

1. b152807f1536141aca4ffac769d51b6075762d45 (to uncover the issue): lowers the PHP versions on which tests (the ones using Docker) are run, so we can detect issues like this one. The change is from PHP 7.3 to PHP 7.2 - I've considered different versions, but since this is the lowest still supported (according to https://www.php.net/supported-versions.php ) I think we can focus on that in tests.
2. 199ff5c23d30d407c59fef22f23796d936530b07: changes the syntax, so that tests can be run on older PHP versions as well

TO DO (before merge):

1. Remove TMP before merging
2. Remove redundant commits

Notes:

1. The code looks different in 7.5 and will require additional changes when merging upstream

TODO:

• Implement feature / fix a bug.
• Implement tests.
• Fix new code according to Coding Standards ($ composer fix-cs).
• Ask for Code Review.

[andrerom]

Seems 5.6 is running into memory:

Package operations: 127 installs, 0 updates, 0 removals

• Installing symfony/polyfill-ctype (v1.13.1): Downloading (100%)
  proc_open(): fork failed - Cannot allocate memory
  The archive may contain identical file names with different capitalization (which fails on case insensitive filesystems)
  Unzip with unzip command failed, falling back to ZipArchive class
  The following exception is caused by a lack of memory or swap, or not having swap configured
  Check https://getcomposer.org/doc/articles/troubleshooting.md#proc-open-fork-failed-errors for details
  PHP Warning: proc_open(): fork failed - Cannot allocate memory in phar:///home/travis/.phpenv/versions/5.6.32/bin/composer/vendor/symfony/console/Application.php on line 952
  Warning: proc_open(): fork failed - Cannot allocate memory in phar:///home/travis/.phpenv/versions/5.6.32/bin/composer/vendor/symfony/console/Application.php on line 952

[ErrorException]
proc_open(): fork failed - Cannot allocate memory

[alongosz]

Seems 5.6 is running into memory:

@mnocon How about running temporarily tests with committed composer.lock? Just to see if on 5.6 it's still working.

Final solution should use PHP 7.1 then.

[mnocon]

@alongosz Sure: a8fff7b

If it passes I'll rework the PR to use PHP7.1. If it fails... I'll see what can be done

[mnocon]

Passing job on 5.6: https://travis-ci.org/ezsystems/ezpublish-kernel/jobs/634269119 (with composer.lock) (Solr job failed because the dependency is not added when running with lock).

I will rework this PR to use PHP7.1 now.

[mnocon]

@alongosz @andrerom please have a look again - all jobs using full setup have been moved to use PHP7.1