Add support of Rails 5

This MR adds support of running GDK on Rails 5.

We are migrating to it now.

The current commands work as before.
New commands for Rails 5: gdk run 5 and gdk run app 5.

doc/set-up-gdk.md

@@ -94,7 +94,11 @@ Check the [GitLab Geo instructions](./howto/geo.md).
 Start GitLab and all required services:
 
 ```sh
+# Rails 4 (current stable)
 gdk run
+
+# Rails 5.0 (next version, we're migrating to it now)
+gdk run rails5
 ```
 
 To start only the databases use:
@@ -106,7 +110,11 @@ gdk run db
 To start only the app (assuming the DBs are already running):
 
 ```sh
+# Rails 4 (current stable)
 gdk run app
+
+# Rails 5.0 (next version, we're migrating to it now)
+gdk run app rails5
 ```
 
 To access GitLab you may now go to http://localhost:3000 in your