Fix PHP 7.4 bug in PPXmlMessage

* ErrorException: Cannot use "parent" when current class scope has no parent
  vendor/paypal/sdk-core-php/lib/PayPal/Core/PPXmlMessage.php:124

  As class has no parent, changed to self instead.

* Update travis config with new php versions

.travis.yml

@@ -4,13 +4,18 @@ matrix:
     include:
         - php: 5.3
           dist: precise
+        - php: 5.4
+          dist: trusty
+        - php: 5.5
+          dist: trusty
 
 php:
-  - 5.4
-  - 5.5
   - 5.6
   - 7.0
   - 7.1
+  - 7.2
+  - 7.3
+  - 7.4snapshot
 
 install:
   - composer install

lib/PayPal/Core/PPXmlFaultMessage.php

@@ -31,7 +31,7 @@ public function init(array $data = array())
         }
 
         if (($first = reset($map)) && !is_array($first) && !is_numeric(key($map))) {
-            parent::init($map, false);
+            self::init($map);
             return;
         }
 

lib/PayPal/Core/PPXmlMessage.php

@@ -121,7 +121,7 @@ public function init(array $map = array(), $isRoot = true)
         }
 
         if (($first = reset($map)) && !is_array($first) && !is_numeric(key($map))) {
-            parent::init($map, false);
+            self::init($map, false);
             return;
         }