don't rely on git ls-files in gemspec file

[sbrun]

I have been building a debian package of em-socket for kali linux and the source package is built on top of the .tar.gz archive (such as those that can be downloaded in the "releases" section of github, aka "git archive" output) and those archives do not contain any ".git" directory. Thus "git ls-files" doesn't work and should be avoided in the gemspec file.
I suggest to use something like this:

  s.files                 = Dir.glob("lib/**/*")
  s.test_files            = Dir.glob("{test,spec,features}/**/*")
  s.executables           = Dir.glob("bin/*").map{ |f| File.basename(f) }