Test against lowest dependencies

Author: alcaeus

.travis.yml

@@ -22,7 +22,7 @@ env:
 matrix:
   include:
     - php: 5.6
-      env: DRIVER_VERSION="1.5.8" SERVER_VERSION="2.6"
+      env: DRIVER_VERSION="1.5.8" SERVER_VERSION="2.6" PREFER_LOWEST="--prefer-lowest"
 
 before_install:
   - sudo apt-key adv --keyserver ${KEY_SERVER} --recv 7F0CEB10
@@ -42,7 +42,7 @@ before_script:
   - if [[ ${TRAVIS_PHP_VERSION:0:2} == "5." ]]; then yes '' | pecl -q install -f mongo-${DRIVER_VERSION}; fi
   - if [[ ${TRAVIS_PHP_VERSION:0:2} == "7." ]]; then pecl install -f mongodb-${DRIVER_VERSION}; fi
   - if [[ ${TRAVIS_PHP_VERSION:0:2} == "7." ]]; then composer require "alcaeus/mongo-php-adapter=${ADAPTER_VERSION}" --ignore-platform-reqs; fi
-  - composer install --dev
+  - composer update ${PREFER_LOWEST}
 
 script:
   - ./vendor/bin/phpunit --coverage-clover=coverage.clover