Fix .rubocop.yml config file to work with current Rubocop versions

Signed-off-by: Sebastian Serth <Sebastian.Serth@student.hpi.de>

.rubocop.yml

@@ -1,4 +1,5 @@
 AllCops:
+  TargetRubyVersion: 2.3
   Exclude:
     - bin/*
     - config/application.rb
@@ -7,11 +8,12 @@ AllCops:
     - db/schema.rb
     - public/uploads/**/*
     - tmp/**/*
-  RunRailsCops: true
+Rails:
+  Enabled: true
 Metrics/LineLength:
   Enabled: false
 require: rubocop-rspec
 Style/Documentation:
   Enabled: false
-Style/SpaceInsideHashLiteralBraces:
+Layout/SpaceInsideHashLiteralBraces:
   EnforcedStyle: no_space