update code to remove deprecated warning in php7.4

Author: vsilva472

.travis.yml

@@ -1,6 +1,9 @@
 language: php
 php:
   - 7.1
+  - 7.2
+  - 7.3
+  - 7.4
 
 before_script:
     - travis_retry composer self-update

composer.json

@@ -9,7 +9,7 @@
         }
     ],
     "require": {
-        "php" : ">=5.6  "
+        "php" : ">=7.1"
     },
     "require-dev": {
       "phpunit/phpunit": "5.5.*"

src/CPF.php

@@ -98,10 +98,10 @@ private function validateDigits ($cpf)
         for ($t = 9; $t < 11; $t++) {
 
             for ($d = 0, $c = 0; $c < $t; $c++) {
-                $d += $cpf{$c} * (($t + 1) - $c);
+                $d += $cpf[$c] * (($t + 1) - $c);
             }
             $d = ((10 * $d) % 11) % 10;
-            if ($cpf{$c} != $d) {
+            if ($cpf[$c] != $d) {
                 return false;
             }
         }