Upgrade rubocop to 0.68.1 and adds rubocop-performance

This should have the same result. The extra gem is because this
rubocop-performance functionality was extracted from the main gem and
needed to be added back in. More info is in rubocop/rubocop#5977

ci/scripts/run-rubocop.sh

@@ -8,4 +8,4 @@ version=`cat Gemfile.lock | grep '    rubocop' | awk -F'[\(*\)]' '{print $2}'`
 gem install rubocop --version $version
 
 echo "Running Rubocop ..."
-/usr/local/bundle/bin/rubocop
+/usr/local/bundle/bin/rubocop --require rubocop-performance

license_finder.gemspec

@@ -59,7 +59,8 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rake'
   s.add_development_dependency 'rspec', '~> 3'
   s.add_development_dependency 'rspec-its'
-  s.add_development_dependency 'rubocop', '~> 0.67.2'
+  s.add_development_dependency 'rubocop', '~> 0.68.1'
+  s.add_development_dependency 'rubocop-performance', '~> 1.0.0'
   s.add_development_dependency 'webmock', '~> 3.5'
 
   # to preserve ruby < 2.2.2 support.