pin Rubocop to 0.60.x

This avoids seemingly "random" test failures on Travis, caused by
a Rubocop update which introduced new rules and/or defaults.

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", "~> 0.49"
+  spec.add_development_dependency "rubocop", "~> 0.60.0"
   spec.add_development_dependency "webmock", "~> 3.0"
 end