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

@@ -98,7 +98,7 @@ Start GitLab and all required services:
 gdk run
 
 # Rails 5.0 (next version, we're migrating to it now)
-gdk run 5
+gdk run rails5
 ```
 
 To start only the databases use:
@@ -114,7 +114,7 @@ To start only the app (assuming the DBs are already running):
 gdk run app
 
 # Rails 5.0 (next version, we're migrating to it now)
-gdk run app 5
+gdk run app rails5
 ```
 
 To access GitLab you may now go to http://localhost:3000 in your