Update Rubocop to fix 2.6-compat

Author: dmke

influxdb.gemspec

@@ -21,6 +21,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler", "~> 1.3"
   spec.add_development_dependency "rake"
   spec.add_development_dependency "rspec", "~> 3.6"
-  spec.add_development_dependency "rubocop", "~> 0.60.0"
+  spec.add_development_dependency "rubocop", "~> 0.61.1"
   spec.add_development_dependency "webmock", "~> 3.0"
 end