BUG Remove bad default value for UserForm Content (#926)

* BUG Remove bad default value for UserForm Content

* BUG Update travis build to use Xenial

.travis.yml

@@ -1,5 +1,10 @@
 language: php
 
+dist: xenial
+
+services:
+  - mysql
+
 env:
   global:
     - COMPOSER_ROOT_VERSION=5.0.x-dev

code/UserForm.php

@@ -107,7 +107,6 @@ trait UserForm
      * @var array Default values of variables when this page is created
      */
     private static $defaults = [
-        'Content' => '$UserDefinedForm',
         'DisableSaveSubmissions' => 0,
         'OnCompleteMessage' => '<p>Thanks, we\'ve received your submission.</p>'
     ];