Add PostgreSQL to Travis builds

.travis.yml

@@ -11,7 +11,7 @@ matrix:
     - php: 5.6
       env: DB=MYSQL PHPCS_TEST=1 PHPUNIT_TEST=1
     - php: 7.0
-      env: DB=MYSQL PHPUNIT_TEST=1
+      env: DB=PGSQL PHPUNIT_TEST=1
     - php: 7.1
       env: DB=MYSQL PHPUNIT_COVERAGE_TEST=1
 
@@ -23,6 +23,7 @@ before_script:
   # Install composer dependencies
   - composer install --prefer-dist
   - composer require --prefer-dist --no-update silverstripe/recipe-cms:1.0.x-dev
+  - if [[ $DB == PGSQL ]]; then composer require --prefer-dist --no-update silverstripe/postgresql:2.0.x-dev; fi
   - composer update
 
 script: