Adjust Open Collective badges to display correctly

After some changes in the OC tiers, we needed to update these badges to reflect the new structure.

README.md

@@ -7,9 +7,10 @@
 [![License](http://img.shields.io/badge/license-BSD-yellowgreen.svg)](LICENSE.md)
 [![Reviewed by Hound](https://img.shields.io/badge/Reviewed_by-Hound-8E64B0.svg)](https://houndci.com)
 
-[![Enthusiasts on Open Collective](https://opencollective.com/solidus/tiers/enthusiast/badge.svg?label=Enthusiasts&color=brightgree)](https://opencollective.com/solidus)
 [![Supporters on Open Collective](https://opencollective.com/solidus/tiers/supporter/badge.svg?label=Supporters&color=brightgree)](https://opencollective.com/solidus)
-[![Ambassadors on Open Collective](https://opencollective.com/solidus/tiers/ambassador/badge.svg?label=Ambassador&color=brightgree)](https://opencollective.com/solidus)
+[![Bronze Partners on Open Collective](https://opencollective.com/solidus/tiers/bronze/badge.svg?label=Bronze Partners&color=brightgree)](https://opencollective.com/solidus)
+[![Silver Partners on Open Collective](https://opencollective.com/solidus/tiers/silver/badge.svg?label=Silver Partners&color=brightgree)](https://opencollective.com/solidus)
+[![Gold Partners on Open Collective](https://opencollective.com/solidus/tiers/gold/badge.svg?label=Gold Partners&color=brightgree)](https://opencollective.com/solidus)
 [![Open Source Helpers](https://www.codetriage.com/solidusio/solidus/badges/users.svg)](https://www.codetriage.com/solidusio/solidus)
 [![Slack](http://slack.solidus.io/badge.svg)](http://slack.solidus.io)