Downgrade PHPUnit to version 6.5.14 and return PHP versions 7.0/7.1

.travis.yml

@@ -5,9 +5,10 @@ install:
   - mkdir -p build/logs
 
 php:
+  - 7.0
+  - 7.1
   - 7.2
   - 7.3
-  - 7.4
 
 script:
   - ./vendor/bin/phpunit

composer.json

@@ -6,7 +6,7 @@
         "php": "^7.0.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^8.5.2",
+        "phpunit/phpunit": "6.5.14",
         "satooshi/php-coveralls": "^1.0.1",
         "codacy/coverage": "^1.0.3"
     },

tests/Emails/EditableEmailTest.php

@@ -20,7 +20,7 @@ class EditableEmailTest extends TestCase
         $editable = new EditableEmail($email);
         $new = $editable->removeFromLocalPart($toRemove);
         $this->assertNotSame($editable, $new);
-        $this->assertStringNotContainsString((string) $new, $toRemove);
+        $this->assertNotContains((string) $new, $toRemove);
         $this->assertSame($email->getDomain(), $new->getDomain());
     }