Use HHVM_VERSION instead of HPHP_VERSION.

Seems there's a chance that `HPHP_VERSION` may be removed one day (see: facebook/hhvm@1632ca1).

Author: whatthejeff

src/CodeCoverage.php

@@ -129,7 +129,7 @@ class PHP_CodeCoverage
     public function __construct(PHP_CodeCoverage_Driver $driver = null, PHP_CodeCoverage_Filter $filter = null)
     {
         if ($driver === null) {
-            if (defined('HPHP_VERSION')) {
+            if (defined('HHVM_VERSION')) {
                 $driver = new PHP_CodeCoverage_Driver_HHVM;
             }
 

src/CodeCoverage/Driver/HHVM.php

@@ -62,7 +62,7 @@ class PHP_CodeCoverage_Driver_HHVM implements PHP_CodeCoverage_Driver
      */
     public function __construct()
     {
-        if (!defined('HPHP_VERSION')) {
+        if (!defined('HHVM_VERSION')) {
             throw new PHP_CodeCoverage_Exception('This driver requires HHVM');
         }
     }

src/CodeCoverage/Report/HTML/Renderer.php

@@ -196,9 +196,9 @@ protected function renderItemTemplate(Text_Template $template, array $data)
      */
     protected function setCommonTemplateVariables(Text_Template $template, PHP_CodeCoverage_Report_Node $node)
     {
-        if (defined('HPHP_VERSION')) {
+        if (defined('HHVM_VERSION')) {
             $runtimeName    = 'HHVM';
-            $runtimeVersion = HPHP_VERSION;
+            $runtimeVersion = HHVM_VERSION;
             $runtimeLink    = 'http://hhvm.com/';
         } else {
             $runtimeName    = 'PHP';