Exclude dotfiles from gem archive

This is especially intended to exclude .rubocop.yml.
Otherwise the rubocop.yml bundled with guard-rubocop may bother user's
.rubocop.yml if the user installed guard-rubocop via bundler in
vendor/bundle and configured to exclude the path in his project's
.rubocop.yml.

guard-rubocop.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = 'https://github.com/yujinakayama/guard-rubocop'
   spec.license       = 'MIT'
 
-  spec.files         = `git ls-files`.split($/)
+  spec.files         = `git ls-files`.split($/).reject { |f| File.basename(f).start_with?('.') }
   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']