Use lemonde instead of mailjet

Mailjet seems to be laggy, hope that LeMonde will be more reliable in the future.

@see https://travis-ci.org/wallabag/wallabag/jobs/56246231

src/Wallabag/CoreBundle/Tests/Controller/WallabagRestControllerTest.php

@@ -105,7 +105,7 @@ class WallabagRestControllerTest extends WallabagTestCase
 
         $this->assertGreaterThanOrEqual(1, count(json_decode($client->getResponse()->getContent())));
 
-        $this->assertContains('Mailjet', $client->getResponse()->getContent());
+        $this->assertContains('Google', $client->getResponse()->getContent());
 
         $this->assertTrue(
             $client->getResponse()->headers->contains(