Add mongodb to travis-ci build

Author: chadicus

.travis.yml

@@ -4,6 +4,8 @@ php:
   - 7.1
   - 7.2
   - nightly
+services: 
+  - mongodb
 env:
   - PREFER_LOWEST="--prefer-lowest --prefer-stable"
   - PREFER_LOWEST=""
@@ -12,6 +14,8 @@ matrix:
   allow_failures:
      - php: nightly
 before_script:
+  - composer self-update
+  - yes '' | pecl install -f mongodb-1.1
   - composer update $PREFER_LOWEST
 script:
   - ./vendor/bin/phpunit --coverage-clover clover.xml