Run --prefer-lowest on lowest PHP version (7.0)

Author: Jean85

.travis.yml

@@ -16,8 +16,8 @@ branches:
 matrix:
     fast_finish: true
     include:
-        - php: 7.1
-          env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest" DEPENDENCIES="doctrine/instantiator:^1.1"
+        - php: 7.0
+          env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest"
 
           # Test the latest stable release
         - php: 7.0