Merge branch 'doc-link-fix' into 'master'

Doc: Fix link to gravatar.com

See merge request !1133

doc/customization/libravatar.md

 # Use Libravatar service with GitLab
 
-GitLab by default supports [Gravatar](gravatar.com) avatar service.
+GitLab by default supports [Gravatar](https://gravatar.com) avatar service.
 Libravatar is a service which delivers your avatar (profile picture) to other websites and their API is
 [heavily based on gravatar](http://wiki.libravatar.org/api/).