Test against PHP 7.1

This also includes a test with against the lowest supported packages

.travis.yml

@@ -5,6 +5,7 @@ php:
   - 5.5
   - 5.6
   - 7.0
+  - 7.1
 
 env:
   - DRIVER_VERSION=stable
@@ -13,6 +14,8 @@ matrix:
   include:
     - php: 5.6
       env: DRIVER_VERSION=stable LEGACY_DRIVER_VERSION=stable
+    - php: 5.5
+      env: DRIVER_VERSION=1.1.0 COMPOSER_FLAGS="--prefer-dist --prefer-lowest"
 
 addons:
   apt:
@@ -23,7 +26,7 @@ addons:
 
 before_script:
   - pecl install -f mongodb-${DRIVER_VERSION}
-  - composer install
+  - composer update ${COMPOSER_FLAGS}
   - if [ "x$LEGACY_DRIVER_VERSION" != "x" ]; then yes '' | pecl -q install -f mongo-${LEGACY_DRIVER_VERSION}; fi
 
 script: