gems: include rubocop-rspec and fix linting

.rubocop.yml

@@ -1,5 +1,6 @@
 require:
 - rubocop-rake
+- rubocop-rspec
 
 AllCops:
   TargetRubyVersion: '2.5'
@@ -89,3 +90,9 @@ Style/SymbolArray:
 
 Style/TrailingCommaInArrayLiteral:
   Enabled: false
+
+RSpec/ExampleLength:
+  Max: 20
+
+RSpec/MultipleExpectations:
+  Max: 20

dyndnsd.gemspec

@@ -42,5 +42,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rspec'
   s.add_development_dependency 'rubocop', '~> 1.6.1'
   s.add_development_dependency 'rubocop-rake', '~> 0.5.1'
+  s.add_development_dependency 'rubocop-rspec', '~> 2.1.0'
   s.add_development_dependency 'solargraph', '~> 0.40.0'
 end

spec/dyndnsd/daemon_spec.rb

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