Exclude older PHP versions from .travis.yml

.travis.yml

@@ -1,7 +1,7 @@
 sudo: false
 language: php
 php:
-  - 5.4
+  - 5.6
 
 env:
   global:
@@ -41,21 +41,17 @@ matrix:
   include:
     - php: 7.0
       env: TC=autoloader;TEST_DAV=0
-    - php: 5.4
+    - php: 5.6
       env: DB=pgsql;TC=litmus-v1
-    - php: 5.4
+    - php: 5.6
       env: DB=sqlite;TC=carddav
-    - php: 5.4
+    - php: 5.6
       env: DB=sqlite;TC=caldav
-    - php: 5.4
-      env: DB=sqlite;TC=syntax;TEST_DAV=0
-    - php: 5.5
-      env: DB=sqlite;TC=syntax;TEST_DAV=0
     - php: 5.6
       env: DB=sqlite;TC=syntax;TEST_DAV=0
     - php: 7.0
       env: DB=sqlite;TC=syntax;TEST_DAV=0
-    - php: 5.4
+    - php: 5.6
       env: DB=sqlite;TC=app:check-code;TEST_DAV=0
 
   fast_finish: true