Fix WagtailErrorPages dead link (#81)

Ironically enough, the build was failing because the link to the WagtailErrorPages repo was returning 404. This is because the owner moved the repo to GitLab.

README.md

@@ -150,7 +150,7 @@ Awesome Wagtail [![Awesome](https://cdn.rawgit.com/sindresorhus/awesome/d7305f38
 
 - [Wagtail Plus](https://github.com/rfosterslo/wagtailplus) - Modular add-ons for Wagtail CMS.
 - [wagtailmenus](https://github.com/rkhleics/wagtailmenus) - An extension for Torchbox's Wagtail CMS to help you manage and render multi-level navigation and simple flat menus in a consistent, flexible way.
-- [Wagtail Error Pages](https://github.com/alexgleason/wagtailerrorpages) - Pretty, smart, customizable error pages for Wagtail.
+- [Wagtail Error Pages](https://gitlab.com/alexgleason/wagtailerrorpages) - Pretty, smart, customizable error pages for Wagtail.
 - [Wagtail Themes](https://github.com/moorinteractive/wagtail-themes) - Site-specific theme loader for Wagtail.
 - [Wagtail Sharing](https://github.com/cfpb/wagtail-sharing) – Easier sharing of Wagtail drafts.
 - [Wagtail Postgres Search Backend](https://github.com/leukeleu/wagtail-pg-search-backend) - PostgreSQL full text search backend for Wagtail CMS.