Add tzinfo-data dev dependency

Needed on Windows:
https://github.com/tzinfo/tzinfo/wiki/Resolving-TZInfo::DataSourceNotFound-Errors
tzinfo is required by (at least) openid_connect gem, which is an "optional"
dependency for kubeclient — users are supposed to bring it themselves if
they use 'oidc' auth — so making tzinfo-data a dev dependency.

kubeclient.gemspec

@@ -30,6 +30,8 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency('mocha', '~> 1.5')
   spec.add_development_dependency 'openid_connect', '~> 1.1'
   spec.add_development_dependency 'net-smtp'
+  # needed on Windows, at least for openid_connect
+  spec.add_development_dependency 'tzinfo-data'
 
   spec.add_dependency 'faraday', '~> 1.1'
   spec.add_dependency 'faraday_middleware', '~> 1.0'