Fix: Resupport 2.2.9 in rubocop

.rubocop.yml

@@ -4,7 +4,7 @@ inherit_from:
 # Files you want to exclude
 AllCops:
-  TargetRubyVersion: 2.3
+  TargetRubyVersion: 2.2
   Exclude:
     - vendor/**/*
     - db/schema.rb

spec/lib/guard/dsl_describer_spec.rb

@@ -30,9 +30,7 @@ RSpec.describe Guard::DslDescriber do
     stub_const 'Guard::Another', class_double('Guard::Plugin')
     # TODO: Reenable rubocop and refactor to +'' one 2.2 compatibility is dropped
-    # rubocop:disable Performance/UnfreezeString - +'' is incompatible with MRI 2.2.9
     @output = String.new
-    # rubocop:enable Performance/UnfreezeString
     # Strip escape sequences
     allow(STDOUT).to receive(:tty?).and_return(false)