You can do this via a rewrite set up on your web server. Assuming that you are using apache you will need to do the following.

1.) In your virtualhost configuration (if you haven't set this up specifically this would be the file /etc/apache2/sites-enabled/000-default.conf) change AllowOverride None to AllowOverride All for your main web root.

2.) Next, enable the rewrite module with the commands:

a2enmod rewrite;
service apache2 restart;

3.) Finally, create an .htaccess file in your web root with the following content. This will redirect any requests for picnicly.com or www.picnicly.com to picnic.ly:

<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteCond %{HTTP_HOST} ^picnicly.com$ [OR]
  RewriteCond %{HTTP_HOST} ^www.picnicly.com$
  RewriteRule (.*)$ http://picnic.ly/$1 [R=301,L]
</IfModule>