Merge branch 'docs/fix-more-urls' into 'master'

Update URLs to better ones

See merge request gitlab-org/gitlab-ce!22931

doc/install/requirements.md

@@ -36,7 +36,7 @@ Please consider using a virtual machine to run GitLab.
 GitLab requires Ruby (MRI) 2.3. Support for Ruby versions below 2.3 (2.1, 2.2) will stop with GitLab 8.13.
 
 You will have to use the standard MRI implementation of Ruby.
-We love [JRuby](http://jruby.org/) and [Rubinius](http://rubini.us/) but GitLab
+We love [JRuby](https://www.jruby.org/) and [Rubinius](https://rubinius.com) but GitLab
 needs several Gems that have native extensions.
 
 ## Hardware requirements

doc/integration/auth0.md

@@ -3,7 +3,7 @@
 To enable the Auth0 OmniAuth provider, you must create an Auth0 account, and an
 application.
 
-1. Sign in to the [Auth0 Console](https://manage.auth0.com). If you need to
+1. Sign in to the [Auth0 Console](https://auth0.com/auth/login). If you need to
 create an account, you can do so at the same link.
 
 1. Select "New App/API".

doc/user/project/import/clearcase.md

 # Migrating from ClearCase
 
-[ClearCase](https://www-03.ibm.com/software/products/en/clearcase/) is a set of
+[ClearCase](https://www.ibm.com/us-en/marketplace/rational-clearcase) is a set of
 tools developed by IBM which also include a centralized version control system
 similar to Git.
 

doc/user/project/integrations/mattermost.md

@@ -4,9 +4,9 @@
 
 To enable Mattermost integration you must create an incoming webhook integration:
 
-1. Sign in to your Mattermost instance
-1. Visit incoming webhooks, that will be something like: https://mattermost.example/your_team_name/integrations/incoming_webhooks/add
-1. Choose a display name, description and channel, those can be overridden on GitLab
+1. Sign in to your Mattermost instance.
+1. Visit incoming webhooks, that will be something like: `https://mattermost.example.com/your_team_name/integrations/incoming_webhooks/add`.
+1. Choose a display name, description and channel, those can be overridden on GitLab.
 1. Save it, copy the **Webhook URL**, we'll need this later for GitLab.
 
 There might be some cases that Incoming Webhooks are blocked by admin, ask your mattermost admin to enable