Travis: test also missing PHP 5.6

Author: f3l1x

Diff for: .travis.yml

@@ -1,5 +1,6 @@
 language: php
 php:
+  - 5.6
   - 7.0
   - 7.1
   - hhvm
@@ -13,6 +14,10 @@ matrix:
     - php: hhvm-nightly
 
   include:
+    - php: 5.6
+      env: COMPOSER_FLAG=--prefer-lowest
+    - php: 5.6
+      env: COMPOSER_FLAG=--prefer-stable
     - php: 7.0
       env: COMPOSER_FLAG=--prefer-lowest
     - php: 7.0