Bump `state_machines-activerecord` to `0.5.1`

What does this MR do?

1. Updates Gemfile.rails5.lock for latest changes in Gemfile on master.
2. Bumps the state_machines-activerecord gem from 0.4.0 to 0.5.1.

Why?

Fixes the error:

% BUNDLE_GEMFILE=Gemfile.rails5 rails console
DEPRECATION WARNING: alias_method_chain is deprecated. Please, use Module#prepend instead. From module, you can access the original method using super. (called from <top (required)> at /projects/gitlab/gdk-ce/gitlab/config/application.rb:5)
rails aborted!
TypeError: compared with non class/module
/var/lib/gems/2.3.0/gems/state_machines-0.5.0/lib/state_machines/integrations/base.rb:27:in `<='
/var/lib/gems/2.3.0/gems/state_machines-0.5.0/lib/state_machines/integrations/base.rb:27:in `block in matches?'
/var/lib/gems/2.3.0/gems/state_machines-0.5.0/lib/state_machines/integrations/base.rb:27:in `any?'
/var/lib/gems/2.3.0/gems/state_machines-0.5.0/lib/state_machines/integrations/base.rb:27:in `matches?'
/var/lib/gems/2.3.0/gems/state_machines-0.5.0/lib/state_machines/integrations.rb:76:in `block in match'
/var/lib/gems/2.3.0/gems/state_machines-0.5.0/lib/state_machines/integrations.rb:76:in `each'
/var/lib/gems/2.3.0/gems/state_machines-0.5.0/lib/state_machines/integrations.rb:76:in `detect'
/var/lib/gems/2.3.0/gems/state_machines-0.5.0/lib/state_machines/integrations.rb:76:in `match'
/var/lib/gems/2.3.0/gems/state_machines-0.5.0/lib/state_machines/machine.rb:511:in `initialize'
/var/lib/gems/2.3.0/gems/state_machines-0.5.0/lib/state_machines/machine.rb:439:in `new'
/var/lib/gems/2.3.0/gems/state_machines-0.5.0/lib/state_machines/machine.rb:439:in `find_or_create'
/var/lib/gems/2.3.0/gems/state_machines-0.5.0/lib/state_machines/macro_methods.rb:517:in `state_machine'
/projects/gitlab/gdk-ce/gitlab/app/models/concerns/has_status.rb:64:in `block in <module:HasStatus>'
...

Why was this MR needed?

Migration to Rails 5.0.

Screenshots (if relevant)

No.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#14286 (closed) and !12841 (closed)