Update .travis.yml

Author: sgiehl

.travis.yml

@@ -31,7 +31,7 @@ env:
     # (PIWIK_TEST_TARGET=maximum_supported_piwik).
     # You can also specify a specific Matomo version
     # (PIWIK_TEST_TARGET=2.16.0-b1).
-    - PIWIK_TEST_TARGET=4.x-dev
+    - PIWIK_TEST_TARGET=5.x-dev
   matrix:
     - TEST_SUITE=UITests MYSQL_ADAPTER=PDO_MYSQL TEST_AGAINST_PIWIK_BRANCH=$PIWIK_TEST_TARGET