Omit test/ dir from built .gem

test_files is deprecated.  It was causing all tests to
be shipped in the .gem file, which is useless bloat:
rubygems/guides#90

kubeclient.gemspec

@@ -16,7 +16,6 @@ Gem::Specification.new do |spec|
 
  spec.files = `git ls-files -z`.split("\x0")
  spec.executables = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
- spec.test_files = spec.files.grep(%r{^(test|spec|features)/})
  spec.require_paths = ['lib']
  spec.required_ruby_version = '>= 2.2.0'