Run tests on PHP 7.4 and simplify test matrix

.travis.yml

@@ -1,25 +1,24 @@
 language: php
 
-php:
-# - 5.3 # requires old distro, see below
-  - 5.4
-  - 5.5
-  - 5.6
-  - 7.0
-  - 7.1
-  - 7.2
-  - 7.3
-  - hhvm # ignore errors, see below
-
 # lock distro so new future defaults will not break the build
 dist: trusty
 
 matrix:
   include:
     - php: 5.3
       dist: precise
+    - php: 5.4
+    - php: 5.5
+    - php: 5.6
+    - php: 7.0
+    - php: 7.1
+    - php: 7.2
+    - php: 7.3
+    - php: 7.4
+    - php: hhvm-3.18
+      install: composer require phpunit/phpunit:^5 --dev --no-interaction # requires legacy phpunit
   allow_failures:
-    - php: hhvm
+    - php: hhvm-3.18
 
 install:
   - composer install --no-interaction

composer.json

@@ -23,6 +23,6 @@
         "react/promise": "^2.7.0 || ^1.2.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.4 || ^5.7 || ^4.8.35"
+        "phpunit/phpunit": "^7.0 || ^6.4 || ^5.7 || ^4.8.35"
     }
 }

tests/FunctionRejectTest.php

@@ -54,6 +54,7 @@ public function testWaitingForPromiseToRejectDoesNotLeaveGarbageCycles()
         }
 
         gc_collect_cycles();
+        gc_collect_cycles(); // clear twice to avoid leftovers in PHP 7.4 with ext-xdebug and code coverage turned on
 
         $promise = Timer\reject(0.01, $this->loop);
         $this->loop->run();