Add various installer versions to Travis build matrix

Author: robbieaverill

.travis.yml

@@ -9,13 +9,13 @@ 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: 7.0
-      env: DB=PGSQL PHPUNIT_TEST=1
+      env: DB=PGSQL INSTALLER_VERSION=4.1.x-dev PHPUNIT_TEST=1
     - php: 7.1
-      env: DB=MYSQL PHPUNIT_COVERAGE_TEST=1
+      env: DB=MYSQL INSTALLER_VERSION=4.2.x-dev PHPUNIT_COVERAGE_TEST=1
     - php: 7.2
-      env: DB=MYSQL PHPUNIT_TEST=1
+      env: DB=MYSQL INSTALLER_VERSION=4.x-dev PHPUNIT_TEST=1
 
 before_script:
 # Init PHP
@@ -25,7 +25,7 @@ before_script:
 
 # Install composer dependencies
   - composer validate
-  - composer require --no-update silverstripe/installer:4.0.x-dev silverstripe/widgets:2.x-dev
+  - composer require --no-update silverstripe/installer:$INSTALLER_VERSION silverstripe/widgets:2.x-dev
   - 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