Updates zend-session to ^2.8.1 for PHP 7.2 compatibility

zend-session 2.8.1 is the first that offers PHP 7.2 compabitibility, and
is thus the first version zend-view can rely on if it wants to provide
7.2 support.

This is a development dependency, so no minor bump is necessary.

Author: weierophinney

composer.json

@@ -39,8 +39,8 @@
         "zendframework/zend-permissions-acl": "^2.6",
         "zendframework/zend-router": "^3.0.1",
         "zendframework/zend-serializer": "^2.6.1",
-        "zendframework/zend-session": "^2.6.2",
         "zendframework/zend-servicemanager": "^2.7.5 || ^3.0.3",
+        "zendframework/zend-session": "^2.8.1",
         "zendframework/zend-uri": "^2.5"
     },
     "suggest": {