Use IO#gets instead of Kernel#gets

serverspec-init command raises error when passing an argument because Kernel#gets read ARGF.

```
% bin/serverspec-init read_from_argf
Select OS type:

  1) UN*X
  2) Windows

Select number: /Users/naoto-takai/Src/external/serverspec/lib/serverspec/setup.rb:55:in `gets': No such file or directory - read_from_argf (Errno::ENOENT)
        from /Users/naoto-takai/Src/external/serverspec/lib/serverspec/setup.rb:55:in `gets'
        from /Users/naoto-takai/Src/external/serverspec/lib/serverspec/setup.rb:55:in `ask_os_type'
        from /Users/naoto-takai/Src/external/serverspec/lib/serverspec/setup.rb:8:in `run'
        from bin/serverspec-init:7:in `<main>'
```