Add prefer lowest dependencies builds

Author: yguedidi

.travis.yml

@@ -11,7 +11,13 @@ cache:
 matrix:
   include:
     - php: 7.1
+    - php: 7.1
+      env:
+        - deps=low
+    - php: nightly
     - php: nightly
+      env:
+        - deps=low
     - php: hhvm
     - php: hhvm-nightly
   # Allow failures until Travis build includes HHVM 3.20.2 which fix the issue with composer (see http://hhvm.com/blog/2017/06/07/hhvm-3-20.html)
@@ -26,7 +32,8 @@ before_install:
 
 install:
   - travis_retry composer require --dev --no-update squizlabs/php_codesniffer
-  - travis_retry composer install --prefer-dist --no-interaction
+  - if [ $deps != low ]; then travis_retry composer update --no-interaction --prefer-dist; fi;
+  - if [ $deps == low ]; then travis_retry composer update --no-interaction --prefer-dist --prefer-lowest; fi;
 
 script:
   - vendor/bin/phpcs