Replace deprecated Twig_Source reference

Author: adamtomat

src/Twig/PrimerLoader.php

@@ -4,7 +4,6 @@
 
 use Rareloop\Primer\Contracts\TemplateProvider;
 use Twig\Loader\LoaderInterface;
-use Twig_Source;
 
 class PrimerLoader implements LoaderInterface
 {
@@ -26,7 +25,7 @@ public function __construct(TemplateProvider $templateProvider)
      */
     public function getSourceContext($name)
     {
-        return new Twig_Source($this->templateProvider->getPatternTemplate($name), $name);
+        return new \Twig\Source($this->templateProvider->getPatternTemplate($name), $name);
     }
 
     /**

tests/Twig/PrimerLoaderTest.php

@@ -51,7 +51,7 @@ public function can_get_source_context()
 
         $source = $loader->getSourceContext('components/misc/header');
 
-        $this->assertInstanceOf(\Twig_Source::class, $source);
+        $this->assertInstanceOf(\Twig\Source::class, $source);
         $this->assertSame('<h1>Hello World</h1>', $source->getCode());
         $this->assertSame('components/misc/header', $source->getName());
     }