Add different installer versions to Travis build matrix

.travis.yml

@@ -14,23 +14,23 @@ env:
 matrix:
   include:
     - php: 5.6
-      env: DB=MYSQL PHPCS_TEST=1 PHPUNIT_TEST=1
+      env: DB=MYSQL INSTALLER_VERSION=4.0.x-dev PHPCS_TEST=1 PHPUNIT_TEST=1
     - php: 7.1
-      env: DB=PGSQL PHPUNIT_TEST=1
+      env: DB=PGSQL INSTALLER_VERSION=4.1.x-dev PHPUNIT_TEST=1
     - php: 7.2
-      env: DB=MYSQL PHPUNIT_COVERAGE_TEST=1
+      env: DB=MYSQL INSTALLER_VERSION=4.2.x-dev PHPUNIT_COVERAGE_TEST=1
     - php: 7.0
-      env: DB=MYSQL BEHAT_TEST=1
+      env: DB=MYSQL INSTALLER_VERSION=4.x-dev BEHAT_TEST=1
   allow_failures:
     - php: 7.0
-      env: DB=MYSQL BEHAT_TEST=1
+      env: DB=MYSQL INSTALLER_VERSION=4.x-dev BEHAT_TEST=1
 
 before_script:
   - phpenv rehash
   - phpenv config-rm xdebug.ini
   - echo 'memory_limit = 2G' >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini
   - composer validate
-  - composer require silverstripe/installer 4.0.x-dev --no-update
+  - composer require silverstripe/installer $INSTALLER_VERSION --no-update
   - if [[ $DB == PGSQL ]]; then composer require --no-update silverstripe/postgresql:2.0.x-dev; fi
   - composer install --prefer-dist --no-interaction --no-progress --no-suggest --optimize-autoloader --verbose --profile
   - echo "SS_BASE_URL=http://localhost:8080/" >> .env