Run tests on PHP 7.4 and simplify test matrix

.travis.yml

@@ -1,27 +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 # requires legacy phpunit & 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: hhvm
-      install: composer require phpunit/phpunit:^5 --dev --no-interaction
+    - 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
 
 sudo: false
 

tests/FunctionalResolverTest.php

@@ -150,6 +150,7 @@ public function testResolveShouldNotCauseGarbageReferencesWhenUsingInvalidNamese
         $this->resolver = $factory->create('255.255.255.255', $this->loop);
 
         gc_collect_cycles();
+        gc_collect_cycles(); // clear twice to avoid leftovers in PHP 7.4 with ext-xdebug and code coverage turned on
 
         $promise = $this->resolver->resolve('google.com');
         unset($promise);

tests/Query/RetryExecutorTest.php

@@ -223,6 +223,8 @@ public function queryShouldNotCauseGarbageReferencesOnSuccess()
         $retryExecutor = new RetryExecutor($executor, 0);
 
         gc_collect_cycles();
+        gc_collect_cycles(); // clear twice to avoid leftovers in PHP 7.4 with ext-xdebug and code coverage turned on
+
         $query = new Query('igor.io', Message::TYPE_A, Message::CLASS_IN);
         $retryExecutor->query($query);