Make the gemspec work on Ruby 2.1

also, excluded the following five files as well:

.codeclimate.yml
.github/workflows/publish.yml
.github/workflows/test.yml
.gitignore
.rubocop.yml

websocket.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |s|
   s.description = 'Universal Ruby library to handle WebSocket protocol'
   s.license     = 'MIT'
 
-  s.files         = `git ls-files`.split("\n").grep_v(/^spec/)
+  s.files         = `git ls-files`.split("\n").reject { |f| f.match(%r{^(spec/|\.)}) }
   s.executables   = `git ls-files -- bin/*`.split("\n").map { |f| File.basename(f) }
   s.require_paths = ['lib']