Add a test layer on lowest dependencies.

This will help identify incompatible changes.

Author: shulard

.travis.yml

@@ -4,6 +4,9 @@ matrix:
   include:
     - php: 5.6
     - php: 7.0
+    - php: 7.0
+      env:
+        - COMPOSER_FLAGS=--prefer-lowest
     - php: 7.1
       env:
         - ENABLE_XDEBUG=true
@@ -30,7 +33,7 @@ before_script:
     fi
 
 script:
-  - composer install
+  - composer update $COMPOSER_FLAGS
   - if [[ $ENABLE_XDEBUG ]]; then
       php vendor/bin/phpunit --coverage-clover clover.xml;
     else