NEW: Add --prefer-lowest to PHP 7.1 build

This build will ensure that older versions of packages that we claim
work with silverstripe/framework do, in fact, work.

‘Composer install’ changed to ‘composer update’ as that’s what’s being
executed in the absence of a composer.lock, and better clarifies intent.

Note that:

 * Support for nikic/php-parser ^2 was lost in 25759ffc5f.

 * Support for monolog/logger < 1.16 was lost in 7ab55a4948

.travis.yml

@@ -27,6 +27,7 @@ matrix:
         - DB=PGSQL
         - PHPCS_TEST=1
         - PHPUNIT_TEST=framework
+        - COMPOSER_INSTALL_ARG="--prefer-lowest"
 
     - php: 7.2
       env:
@@ -88,7 +89,7 @@ before_script:
   - composer require silverstripe/recipe-testing:^1 silverstripe/recipe-core:4.x-dev silverstripe/admin:1.x-dev silverstripe/versioned:1.x-dev --no-update
   - if [[ $PHPUNIT_TEST == cms ]]; then composer require silverstripe/recipe-cms:4.x-dev --no-update; fi
   - if [[ $PHPCS_TEST ]]; then composer global require squizlabs/php_codesniffer:^3 --prefer-dist --no-interaction --no-progress --no-suggest -o; fi
-  - composer install --prefer-source --no-interaction --no-progress --no-suggest --optimize-autoloader --verbose --profile $COMPOSER_INSTALL_ARG
+  - composer update --prefer-source --no-interaction --no-progress --no-suggest --optimize-autoloader --verbose --profile $COMPOSER_INSTALL_ARG
 
 # Log constants to CI for debugging purposes
   - php ./tests/dump_constants.php

composer.json

@@ -28,8 +28,8 @@
         "league/csv": "^8 || ^9",
         "league/flysystem": "~1.0.12",
         "m1/env": "^2.1",
-        "monolog/monolog": "~1.11",
-        "nikic/php-parser": "^2 || ^3 || ^4",
+        "monolog/monolog": "~1.16",
+        "nikic/php-parser": "^3 || ^4",
         "psr/container": "1.0.0",
         "psr/container-implementation": "1.0.0",
         "silverstripe/config": "^1@dev",