[ruby/io-nonblock] Fix ls-files matching regexp

See rubygems/rubygems@8a81183236.

https://github.com/ruby/io-nonblock/commit/e14f7952c1

ext/io/nonblock/io-nonblock.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |spec|
   spec.metadata["source_code_uri"] = spec.homepage
 
   spec.files         = Dir.chdir(File.expand_path('..', __FILE__)) do
-    `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
+    `git ls-files -z`.split("\x0").reject { |f| f.match(%r{\A(?:test|spec|features)/}) }
   end
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }