Fix cache clear of image derivatives and wildcards for varnish

#4430 

This is paired with a patch to "acquia_purge" in our acquia repo:
```patch
diff --git a/acquia_purge.module b/acquia_purge.module
index c27e35e..ebdbe84 100644
--- a/acquia_purge.module
+++ b/acquia_purge.module
@@ -110,9 +110,24 @@ function acquia_purge_menu() {
 /**
  * Implements hook_expire_cache().
  */
-function acquia_purge_expire_cache($paths) {
+function acquia_purge_expire_cache($raw_paths, $wildcards = array()) {
   static $patterns;

+  $paths = array();
+  foreach ($raw_paths as $internal_path => $absolute_url) {
+    // Check if wildcard is enabled for this URL.
+    if(!empty($wildcards[$internal_path])){
+      //Break up aliased and internal paths for older acquia_purge
+      $paths[] = $internal_path."(.*)";
+      $paths[] = $absolute_url."(.*)";
+    }else{
+      //Break up aliased and internal paths for older acquia_purge
+      $paths[] = $internal_path;
+      $paths[] = $absolute_url;
+    }
+  }
+  $paths = array_unique($paths);
+
   // Ask our built-in diagnostics system to preliminary find issues that are so
   // risky we can expect problems. Everything with ACQUIA_PURGE_SEVLEVEL_ERROR
   // will cause purging to cease and we've got to tell the end user.
```