[ticket/14972] Update tests matrix to reflect bumping PHP version requirement

PHPBB3-14972

.travis.yml

@@ -3,19 +3,15 @@ sudo: required
 
 matrix:
   include:
-    - php: 5.5
+    - php: 5.6
       env: DB=none;NOTESTS=1
-    - php: 5.5
-      env: DB=mysqli
-    - php: 5.5
-      env: DB=mysql
-    - php: 5.5
+    - php: 5.6
       env: DB=mariadb
-    - php: 5.5
+    - php: 5.6
       env: DB=postgres
-    - php: 5.5
+    - php: 5.6
       env: DB=sqlite3
-    - php: 5.5
+    - php: 5.6
       env: DB=mysqli;SLOWTESTS=1
     - php: 5.6
       env: DB=mysqli