diff --git a/.travis.yml b/.travis.yml
index 02227d0..e1ef244 100644
--- a/.travis.yml
+++ b/.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
diff --git a/composer.json b/composer.json
index 87e9d8c..d6c11af 100644
--- a/composer.json
+++ b/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"
     },
diff --git a/tests/Emails/EditableEmailTest.php b/tests/Emails/EditableEmailTest.php
index 8471e08..8049bd1 100644
--- a/tests/Emails/EditableEmailTest.php
+++ b/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());
     }