Update to Rails 5.1

We need to update to Rails 5.2 because of the security issue:

https://groups.google.com/forum/#!topic/rubyonrails-security/IsQKvDqZdKw

But this is not an easy update, so let's try to update in small steps.

Gemfile

@@ -10,7 +10,7 @@ if Gem::Version.new(Bundler::VERSION) < Gem::Version.new('1.8.4')
 end
 
 # as web framework
-gem 'rails', '~> 5.0.7'
+gem 'rails', '~> 5.1.0'
 
 # Use Puma as the app server
 gem 'puma', '~> 3.0'