Fix RuboCop 0.40 linter errors (#1722)

These errors are breaking the build, which seems to use RuboCop 0.40 [1]
despite the Gemfile.lock pinning rubocop to 0.38.

New lints that I am updating the code style to reflect:

- Style/EmptyCaseCondition: Do not use empty case condition, instead use
  an if expression.

- Style/MultilineArrayBraceLayout: Closing array brace must be on the
  same line as the last array element when opening brace is on the same
  line as the first array element.

- Style/MultilineHashBraceLayout: Closing hash brace must be on the same
  line as the last hash element when opening brace is on the same line
  as the first hash element.

- Style/MultilineMethodCallBraceLayout: Closing method call brace must
  be on the line after the last argument when opening brace is on a
  separate line from the first argument.

[1] https://github.com/bbatsov/rubocop/releases/tag/v0.40.0

lib/active_model/serializer/include_tree.rb

@@ -95,12 +95,11 @@ module ActiveModel
 
       def [](key)
         # TODO(beauby): Adopt a lazy caching strategy for generating subtrees.
-        case
-        when @hash.key?(key)
+        if @hash.key?(key)
           self.class.new(@hash[key])
-        when @hash.key?(:*)
+        elsif @hash.key?(:*)
           self.class.new(@hash[:*])
-        when @hash.key?(:**)
+        elsif @hash.key?(:**)
           self.class.new(:** => {})
         else
           nil