Test against PHP 7.3 and add forward compatibility with PHPUnit 7

Additionally, use legacy PHPUnit 5 for legacy HHVM on Travis CI as per
clue/socket-raw#42

Author: clue

.travis.yml

@@ -7,7 +7,9 @@ php:
   - 5.6
   - 7.0
   - 7.1
-  - hhvm # ignore errors, see below
+  - 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
@@ -16,14 +18,16 @@ matrix:
   include:
     - php: 5.3
       dist: precise
+    - php: hhvm
+      install: composer require phpunit/phpunit:^5 --dev --no-interaction
   allow_failures:
     - php: hhvm
 
 sudo: false
 
 install:
   - composer install --no-interaction
-  
+
 script:
   - vendor/bin/phpunit --coverage-text
   - php examples/13-benchmark-throughput.php

composer.json

@@ -10,7 +10,7 @@
         "react/stream": "^1.0 || ^0.7.6"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.4 || ^5.7 || ^4.8.35",
+        "phpunit/phpunit": "^7.0 || ^6.4 || ^5.7 || ^4.8.35",
         "sebastian/environment": "^3.0 || ^2.0 || ^1.0"
     },
     "autoload": {

phpunit.xml.dist

@@ -8,7 +8,6 @@
          convertWarningsToExceptions="true"
          processIsolation="false"
          stopOnFailure="false"
-         syntaxCheck="false"
          bootstrap="vendor/autoload.php"
 >
     <testsuites>