Fixes curly braces deprecation notices for PHP 7.4 (#6)

* Fixes curly braces deprecation notices for PHP 7.4 (see https://wiki.php.net/rfc/deprecate_curly_braces_array_access)
* Adds PHP 7.4 to test targets for Travis CI
* Fixes wrong property access

.travis.yml

@@ -6,6 +6,7 @@ php:
 - '7.1'
 - '7.2'
 - '7.3'
+- '7.4'
 
 before_script:
 - composer self-update

lib/avro/protocol.php

@@ -63,7 +63,7 @@ function real_parse($avro) {
     if (!is_null($avro["messages"])) {
       foreach ($avro["messages"] as $messageName => $messageAvro) {
         $message = new AvroProtocolMessage($messageName, $messageAvro, $this);
-        $this->messages{$messageName} = $message;
+        $this->messages[$messageName] = $message;
       }
     }
   }
@@ -91,12 +91,12 @@ class AvroProtocolMessage
   public function __construct($name, $avro, $protocol)
   {
     $this->name = $name;
-    $this->request = new AvroRecordSchema(new AvroName($name, null, $protocol->namespace), null, $avro{'request'}, $protocol->schemata, AvroSchema::REQUEST_SCHEMA);
+    $this->request = new AvroRecordSchema(new AvroName($name, null, $protocol->namespace), null, $avro['request'], $protocol->schemata, AvroSchema::REQUEST_SCHEMA);
 
     if (array_key_exists('response', $avro)) {
-      $this->response = $protocol->schemata->schema_by_name(new AvroName($avro{'response'}, $protocol->namespace, $protocol->namespace));
+      $this->response = $protocol->schemata->schema_by_name(new AvroName($avro['response'], $protocol->namespace, $protocol->namespace));
       if ($this->response == null)
-        $this->response = new AvroPrimitiveSchema($avro{'response'});
+        $this->response = new AvroPrimitiveSchema($avro['response']);
     }
   }
 }