diff --git a/.travis.yml b/.travis.yml
index 14f5988..85494d6 100755
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,7 +6,3 @@ php:
   - 7.3
   - 7.2
   - 7.1
-  - 7.0
-  - 5.6
-  - 5.5
-  - 5.4
diff --git a/README.md b/README.md
index bbf9d70..3367fa8 100644
--- a/README.md
+++ b/README.md
@@ -34,7 +34,7 @@ $converter->parseString('<h1 id="md">Heading</h1>');
 How to Install
 --------
 
-This library package requires `PHP 5.4` or later.<br>
+This library package requires `PHP 7.1` or later.<br>
 Install [Composer](http://getcomposer.org/doc/01-basic-usage.md#installation) and run the following command to get the latest version:
 
 ```sh
diff --git a/composer.json b/composer.json
index 040b1e9..9376798 100644
--- a/composer.json
+++ b/composer.json
@@ -19,10 +19,10 @@
         }
     ],
     "require": {
-        "php": ">=5.4.0"
+        "php": ">=7.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.8"
+        "phpunit/phpunit": "^9.5"
     },
     "autoload": {
         "psr-4": {
diff --git a/test/ConverterExtraTest.php b/test/ConverterExtraTest.php
index 69895ff..0e778b3 100644
--- a/test/ConverterExtraTest.php
+++ b/test/ConverterExtraTest.php
@@ -12,7 +12,7 @@ class ConverterExtraTest extends ConverterTestCase
 
     /* UTILS
      *************************************************************************/
-    public function setUp()
+    public function setUp(): void
     {
         $this->converter = new ConverterExtra;
     }
diff --git a/test/ConverterTest.php b/test/ConverterTest.php
index 0abfc3e..5c77596 100644
--- a/test/ConverterTest.php
+++ b/test/ConverterTest.php
@@ -12,7 +12,7 @@ class ConverterTest extends ConverterTestCase
 
     /* UTILS
      *************************************************************************/
-    public function setUp()
+    public function setUp(): void
     {
         $this->converter = new Converter;
     }
diff --git a/test/ConverterTestCase.php b/test/ConverterTestCase.php
index 1930f8d..3134d67 100644
--- a/test/ConverterTestCase.php
+++ b/test/ConverterTestCase.php
@@ -3,15 +3,19 @@
 namespace Test\Markdownify;
 
 use Markdownify\Converter;
+use PHPUnit\Framework\TestCase;
 
 require_once(__DIR__ . '/../vendor/autoload.php');
 
-class ConverterTestCase extends \PHPUnit_Framework_TestCase
+class ConverterTestCase extends TestCase
 {
 
 
     /* ATTRIBUTES
      *************************************************************************/
+    /**
+     * @var Converter
+     */
     protected $converter;
 
 
@@ -554,10 +558,10 @@ public function testResetState()
         $converter = new Converter();
         $bqOutput = $converter->parseString($blockquote);
 
-        $this->assertContains('>', $bqOutput);
+        $this->assertStringContainsString('>', $bqOutput);
 
         $lbOutput = $converter->parseString($linebreaks);
 
-        $this->assertNotContains('>', $lbOutput);
+        $this->assertStringNotContainsString('>', $lbOutput);
     }
 }