minor #17534 [ClassLoader] suggest apcu polyfill for hhvm (Tobion)

This PR was merged into the 3.0 branch.

Discussion
----------

[ClassLoader] suggest apcu polyfill for hhvm

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/pull/17524#issuecomment-174533680
| License       | MIT
| Doc PR        | -

Same approach as in https://github.com/symfony/symfony/pull/17530/files#diff-877e1eb36c14b7b8dc89a910744ebfcdR31

Commits
-------

714fcc6 suggest apcu polyfill as apcu_* function are not yet available on hhvm

src/Symfony/Component/ClassLoader/composer.json

@@ -22,6 +22,9 @@
     "require-dev": {
         "symfony/finder": "~2.8|~3.0"
     },
+    "suggest": {
+        "symfony/polyfill-apcu": "For using ApcClassLoader on HHVM"
+    },
     "autoload": {
         "psr-4": { "Symfony\\Component\\ClassLoader\\": "" },
         "exclude-from-classmap": [