Merge branch '2.7' into 2.8

* 2.7:
  [travis] Don't use parallel on HHVM
  [appveyor] Ignore STATUS_HEAP_CORRUPTION errors on Windows
  Skip some tests on HHVM due to a PHPunit bug
  Use the Trusty Travis infrastructure for HHVM builds
  Add 3.1 to  PR template branch row, remove 2.3
  Improve memory efficiency

Author: nicolas-grekas

Tests/DataCollector/DumpDataCollectorTest.php

@@ -128,7 +128,7 @@ public function testFlush()
         $line = __LINE__ - 1;
 
         ob_start();
-        $collector = null;
+        $collector->__destruct();
         if (PHP_VERSION_ID >= 50400) {
             $this->assertSame("DumpDataCollectorTest.php on line {$line}:\n456\n", ob_get_clean());
         } else {