Add test against lowest dependencies to matrix

.travis.yml

@@ -6,7 +6,6 @@ services:
   - mongodb
 
 php:
-  - 5.6
   - 7.0
   - 7.1
   - 7.2
@@ -26,8 +25,12 @@ addons:
 matrix:
   fast_finish: true
   include:
+    # Test against legacy driver to ensure validity of the test suite
     - php: 5.6
-      env: DRIVER_VERSION=stable LEGACY_DRIVER_VERSION=stable COMPOSER_FLAGS="--prefer-dist --prefer-lowest"
+      env: DRIVER_VERSION=stable LEGACY_DRIVER_VERSION=stable
+    # Test against set of lowest dependencies
+    - php: 5.6
+      env: DRIVER_VERSION="1.2.0" COMPOSER_FLAGS="--prefer-dist --prefer-lowest"
       addons:
         apt:
           sources:
@@ -35,8 +38,6 @@ matrix:
               key_url: "https://www.mongodb.org/static/pgp/server-3.0.asc"
             - "mongodb-upstart"
           packages: ['mongodb-org-server']
-    - php: 7.1
-      env: DRIVER_VERSION="1.3.0"
 
 before_install:
   - pecl install -f mongodb-${DRIVER_VERSION}