Disable unhelpful RuboCop rule

RuboCop's `Lint/AmbiguousBlockAssociation` rule forbids this construct:

```ruby
set :foo, -> { "bar" }
```

It apparently wants:

```ruby
set(:foo, -> { "bar" })
```

This goes against the grain of Capistrano's DSL, so disable it.

.rubocop.yml

@@ -3,6 +3,9 @@ AllCops:
   DisplayStyleGuide: true
   TargetRubyVersion: 2.0
 
+Lint/AmbiguousBlockAssociation:
+  Enabled:
+    false
 Metrics/BlockLength:
   Exclude:
     - "spec/**/*"