refactor(migrations) : split 'tasks' down migration for each column to avoid breaking tests in sqlite

Here is an example of how to fix the possible issue refered by #39

Author: Arkanius

database/migrations/2017_08_26_083622_alter_tasks_table_add_notifications_fields.php

@@ -28,6 +28,9 @@ public function down()
     {
         Schema::table('tasks', function (Blueprint $table) {
             $table->dropColumn('notification_phone_number');
+        });
+
+        Schema::table('tasks', function (Blueprint $table) {
             $table->dropColumn('notification_slack_webhook');
         });
     }