title: In Laravel 5, migration to drop composed unique key doesn't work
body: <p>Trying to make a migration to drop composed unique key fails without errors.</p>
  
  <p>I've created a migration with <code>php artisan make:migration</code> and edited the code. So I have this</p>
  
  <pre><code>public function up()
  {
      Schema::table('ques_trilha_itens', function (Blueprint $table) {
          $table-&gt;dropUnique('trilha_itens_trilha_id_questao_id_unique');
      });
  }
  
  /**
   * Reverse the migrations.
   *
   * @return void
   */
  public function down()
  {
      Schema::table('ques_trilha_itens', function (Blueprint $table) {
          $table-&gt;unique(['trilha_id', 'questao_id']);
      });
  }
  </code></pre>
  
  <p>The string <code>'trilha_itens_trilha_id_questao_id_unique'</code> is the one that is displayed as the composed unique key in MySQL. So I think that string is to be used do drop the two composed keys.</p>
  
  <p>But when running <code>php artisan migrate</code>, nothing happens, no error messages, and the migration is not executed.</p>
  
  <p>I tried substitute the string in <code>dropUnique</code> to give the table's name as the first term (<code>'ques_trilha_itens_trilha_id_questao_id_unique'</code>) and nothing.</p>
  
  <p>Is something I'm missing?</p>
  
  <p><strong>UPDATE</strong>:</p>
  
  <p>MySQL command <code>SHOW CREATE TABLE ques_trilha_itens</code> give:</p>
  
  <pre><code>CREATE TABLE `ques_trilha_itens` (
    `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
    `trilha_id` int(10) unsigned NOT NULL,
    `questao_id` int(10) unsigned NOT NULL,
    `primeiro_item` tinyint(1) NOT NULL,
    `item_principal_id` int(10) unsigned DEFAULT NULL,
    `created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
    `updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
    `chave` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
    `deleted_at` timestamp NULL DEFAULT NULL,
    PRIMARY KEY (`id`),
    UNIQUE KEY `trilha_itens_trilha_id_questao_id_unique` (`trilha_id`,`questao_id`),
    UNIQUE KEY `ques_trilha_itens_chave_unique` (`chave`),
    KEY `trilha_itens_questao_id_foreign` (`questao_id`),
    KEY `ques_trilha_itens_item_principal_id_foreign` (`item_principal_id`),
    CONSTRAINT `ques_trilha_itens_item_principal_id_foreign` FOREIGN KEY (`item_principal_id`) REFERENCES `ques_trilha_itens` (`id`) ON DELETE SET NULL,
    CONSTRAINT `trilha_itens_trilha_id_foreign` FOREIGN KEY (`trilha_id`) REFERENCES `ques_trilhas` (`id`)
  ) ENGINE=InnoDB AUTO_INCREMENT=43 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
  </code></pre>