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

test/adapter/json_test.rb

@@ -33,9 +33,10 @@ module ActiveModelSerializers
 
         assert_equal({
           id: 1,
-          reviews: [{ id: 1, body: 'ZOMG A COMMENT' },
-                    { id: 2, body: 'ZOMG ANOTHER COMMENT' }
-                  ],
+          reviews: [
+            { id: 1, body: 'ZOMG A COMMENT' },
+            { id: 2, body: 'ZOMG ANOTHER COMMENT' }
+          ],
           writer: { id: 1, name: 'Steve K.' },
           site: { id: 1, name: 'My Blog!!' }
           }, adapter.serializable_hash[:post])