gem: adopt frozen string literals

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'