Tell Rubocop to enforce semantic block delimiters

N.B. This is a new feature of Rubocop 0.30.0:
https://github.com/bbatsov/rubocop/blob/master/CHANGELOG.md

> The `semantic` style enforces braces around functional blocks, where
> the primary purpose of the block is to return a value and do..end
> for procedural blocks, where the primary purpose of the block is its
> side-effects.

Hound CI currently uses Rubocop 0.29.0. To ensure Hound does not flag
our semantic block style, we need to keep the `Blocks` cop config in
our `.rubocop.yml` until they upgrade.

[ci skip]

Signed-off-by: Alex Coles <alex@alexbcoles.com>

.rubocop.yml

@@ -30,6 +30,12 @@ Attr:
 BlockNesting:
   Enabled: false
 
+BlockDelimiters:
+  Enabled: true
+  EnforcedStyle: semantic
+  IgnoredMethods:
+    - default_scope
+
 Blocks:
   Enabled: false