Remove mailgun

Mailgun only provides a testing tier. When this tier's limit runs out, you'll need to pay to keep using it.

ref: https://www.mailgun.com/pricing/
```
Try us out! Get 5,000 free emails per month for 3 months, then only pay for what you send

Pay As You Grow
```

README.md

@@ -272,7 +272,6 @@ API | Description | Auth | HTTPS | CORS |
 | [Google Analytics](https://developers.google.com/analytics/) | Collect, configure and analyze your data to reach the right audience | `OAuth` | Yes | Unknown |
 | [Instatus](https://instatus.com/help/api) | Post to and update maintenance and incidents on your status page through an HTTP REST API | `apiKey` | Yes | Unknown |
 | [Mailchimp](https://mailchimp.com/developer/) | Send marketing campaigns and transactional mails | `apiKey` | Yes | Unknown |
-| [mailgun](https://www.mailgun.com/) | Email Service | `apiKey` | Yes | Unknown |
 | [mailjet](https://www.mailjet.com/) | Marketing email can be sent and mail templates made in MJML or HTML can be sent using API | `apiKey` | Yes | Unknown |
 | [markerapi](http://www.markerapi.com/) | Trademark Search | No | No | Unknown |
 | [ORB Intelligence](https://api.orb-intelligence.com/docs/) | Company lookup | `apiKey` | Yes | Unknown |