Add ArcanistRaggedClassTreeEdgeXHPASTLinterRule to Phutil linter map

Summary: Fixes T12555.

Test Plan:
Added this class to the codebase and ran `arc liberate`:
```
<?php

class FooBar {
    public static function doTheFoo() {
        return 'foobar';
    }
}
```

Ran `arc lint` and observed this warning:
```
   Warning  (XHP87) Class Not `abstract` Or `final`
    This class is neither `final` nor `abstract`, and does not have a
    docblock marking it `@concrete-extensible`.

               1 <?php
               2
    >>>        3 class FooBar {
               4     public static function doTheFoo() {
               5         return "foobar";
               6     }
```

Added a `final` modifier to `FooBar`'s declaration and observed the warning went away.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12555

Differential Revision: https://secure.phabricator.com/D17787

src/lint/linter/standards/phutil/ArcanistPhutilXHPASTLinterStandard.php

@@ -58,10 +58,12 @@ final class ArcanistPhutilXHPASTLinterStandard
   public function getLinterSeverityMap() {
     $advice  = ArcanistLintSeverity::SEVERITY_ADVICE;
     $error   = ArcanistLintSeverity::SEVERITY_ERROR;
+    $warning = ArcanistLintSeverity::SEVERITY_WARNING;
 
     return array(
       ArcanistTodoCommentXHPASTLinterRule::ID         => $advice,
       ArcanistCommentSpacingXHPASTLinterRule::ID      => $error,
+      ArcanistRaggedClassTreeEdgeXHPASTLinterRule::ID => $warning,
     );
   }