gem: adopt frozen string literals

spec/daemon_spec.rb

@@ -1,4 +1,6 @@
-require 'spec_helper'
+# frozen_string_literal: true
+
+require_relative 'spec_helper'
 
 describe Dyndnsd::Daemon do
   include Rack::Test::Methods

spec/spec_helper.rb

@@ -1,8 +1,10 @@
+# frozen_string_literal: true
 
 require 'rubygems'
 require 'bundler/setup'
 require 'rack/test'
 
 require 'dyndnsd'
-require 'support/dummy_database'
-require 'support/dummy_updater'
+
+require_relative 'support/dummy_database'
+require_relative 'support/dummy_updater'

spec/support/dummy_database.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 
 require 'forwardable'
 

spec/support/dummy_updater.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 
 module Dyndnsd
   module Updater