Backport Ref<T> to 3.30 (#113)

* Backport Ref<T> to 3.30

* Remove hhvm-nightly from the tests

HHVM 4.41-dev is smiling upon thou hsl 3.30 😜.

* (Re)target hhvm 3.30-latest in tests

Travis picks the newest stable hhvm is you don't say anything.

.travis.yml

@@ -1,7 +1,6 @@
 language: php
 php:
- - hhvm
- - hhvm-nightly
+ - hhvm-3.30
 matrix:
   allow_failures:
   - php: hhvm

src/Ref.php

@@ -0,0 +1,28 @@
+<?hh // strict
+/*
+ *  Copyright (c) 2004-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the MIT license found in the
+ *  LICENSE file in the root directory of this source tree.
+ *
+ */
+
+namespace HH\Lib;
+
+/** Wrapper class for getting object (byref) semantics for a value type.
+ *
+ * This is especially useful for mutating values outside of a lambda's scope.
+ *
+ * In general, it's preferable to refactor to use return values or `inout`
+ * parameters instead of using this class - however, a `Ref` of a Hack array
+ * is generally preferable to a Hack collection - e.g. prefer `Ref<vec<T>>`
+ * over `Vector<T>`.
+ *
+ * `C\reduce()` and `C\reduce_with_key()` can also be used in some situations
+ * to avoid this class.
+ */
+final class Ref<T> {
+  <<__RxShallow>>
+  public function __construct(public T $value) {}
+}