changes display value of checkboxes on discounts page to inline-block. Fixes width issue.

In the WP 3.8 Admin themes, the checkboxes on the discount pages were not showing because their CSS `display` property was set to `inline`. Changing this to `inline-block` fixes it while retaining backwards compatibility with EDD users running older WP versions.

![discount_codes__mailchimp_for_wordpress__wordpress_20131223_201736](https://f.cloud.github.com/assets/885856/1802708/75896574-6c08-11e3-88af-ce1007e12ce5.jpg)