Add PHP 7.4 support

- Looking at the PHP 7.4 Backward Incompatible Changes and the repositories code, there is no reason to block this version
- All PhpUnit tests run succesful in PHP 7.4

Author: youwe-petervanderwal

.travis.yml

@@ -4,6 +4,7 @@ php:
   - 7.1
   - 7.2
   - 7.3
+  - 7.4
 
 env:
   - ES_URL=http://127.0.0.1:9200

composer.json

@@ -3,7 +3,7 @@
     "description": "A non-blocking ElasticSearch client for PHP based on Amp.",
     "license": "MIT",
     "require": {
-        "php": "~7.1.0|~7.2.0|~7.3.0",
+        "php": "~7.1.0|~7.2.0|~7.3.0|~7.4.0",
         "ext-json": "*",
         "amphp/artax": "^3.0",
         "amphp/amp": "^2.1"