Travis: test DokuWiki old-stable only on PHP 5.6

More recent PHP versions fail because of an incompatible PHPUnit version.

Author: michitux

.travis.yml

@@ -8,7 +8,10 @@ php:
 env:
   - DOKUWIKI=master
   - DOKUWIKI=stable
-  - DOKUWIKI=old-stable
+matrix:
+  include:
+    - php: "5.6"
+      env: DOKUWIKI=old-stable
 before_install: wget https://raw.github.com/splitbrain/dokuwiki-travis/master/travis.sh
 install: sh travis.sh
 script: cd _test && phpunit --stderr --group plugin_include