setup mongodb on travis

.travis.yml

@@ -12,6 +12,9 @@ cache:
 env:
   - COMPOSER_FLAGS="--prefer-stable --prefer-dist"
 
+services:
+  - mongodb
+
 php:
   - 7.1
   - 7.2
@@ -37,7 +40,7 @@ before_install:
   - composer self-update --stable --no-progress
 
 install:
-  - travis_retry composer require alcaeus/mongo-php-adapter $COMPOSER_GLOBAL_FLAGS $COMPOSER_FLAGS --ignore-platform-reqs
+  - travis_retry composer require alcaeus/mongo-php-adapter $COMPOSER_GLOBAL_FLAGS $COMPOSER_FLAGS
   - travis_retry composer update $COMPOSER_FLAGS --no-interaction --no-scripts --no-progress
   - if [[ $TEST_VERSION ]]; then travis_retry composer require php-coveralls/php-coveralls $COMPOSER_FLAGS --no-interaction --no-scripts --no-progress ; fi