* add missing rubocop-performance dependency to gemspec

* set targeted ruby version for rubocop to 2.4

.rubocop.yml

@@ -2,7 +2,7 @@ require:
   - rubocop-performance
 
 AllCops:
-  TargetRubyVersion: 2.3
+  TargetRubyVersion: 2.4
 
 Metrics/LineLength:
   Max: 120

capistrano-logtail.gemspec

@@ -24,4 +24,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'bundler'
   spec.add_development_dependency 'rake'
   spec.add_development_dependency 'rubocop'
+  spec.add_development_dependency 'rubocop-performance'
 end