Add test with lowest dependencies

Author: jurv

.travis.yml

@@ -14,11 +14,12 @@ env:
         - SELENIUM_VERSION=3.6.0
     matrix:
         - DEPS="" PROFILE=default
-#        - DEPS="--prefer-lowest" PROFILE=default
+        - DEPS="--prefer-lowest" PROFILE=default
         - DEPS="" PROFILE=symfony2
-#        - DEPS="--prefer-lowest" PROFILE=symfony2
+        - DEPS="--prefer-lowest" PROFILE=symfony2
 
 php:
+    - 5.6
     - 7.0
     - 7.1
     - 7.2