Support Windows when not using backtick execution

Fixes https://github.com/jekyll/jekyll/issues/2556

/cc @piki
I think `master` is broken on 1.8...

```text
cp tmp/i686-darwin13.2.0/posix_spawn_ext/1.8.7/posix_spawn_ext.bundle lib/posix_spawn_ext.bundle
/Users/parker/code/posix-spawn/lib/posix-spawn.rb:1:in `require': /Users/parker/code/posix-spawn/lib/posix/spawn.rb:241: syntax error, unexpected tSYMBEG, expecting tAMPER (SyntaxError)
          ::Kernel::exec(*argv, :close_others=>false)
                                 ^
	from /Users/parker/code/posix-spawn/lib/posix-spawn.rb:1
	from /Users/parker/code/posix-spawn/test/test_backtick.rb:2:in `require'
	from /Users/parker/code/posix-spawn/test/test_backtick.rb:2
	from /Users/parker/.rbenv/versions/1.8.7-p375/lib/ruby/gems/1.8/gems/rake-10.3.2/lib/rake/rake_test_loader.rb:15:in `require'
	from /Users/parker/.rbenv/versions/1.8.7-p375/lib/ruby/gems/1.8/gems/rake-10.3.2/lib/rake/rake_test_loader.rb:15
	from /Users/parker/.rbenv/versions/1.8.7-p375/lib/ruby/gems/1.8/gems/rake-10.3.2/lib/rake/rake_test_loader.rb:4:in `select'
	from /Users/parker/.rbenv/versions/1.8.7-p375/lib/ruby/gems/1.8/gems/rake-10.3.2/lib/rake/rake_test_loader.rb:4
rake aborted!
Command failed with status (1): [ruby -I"lib" -I"/Users/parker/.rbenv/versions/1.8.7-p375/lib/ruby/gems/1.8/gems/rake-10.3.2/lib" "/Users/parker/.rbenv/versions/1.8.7-p375/lib/ruby/gems/1.8/gems/rake-10.3.2/lib/rake/rake_test_loader.rb" "test/test_backtick.rb" "test/test_child.rb" "test/test_popen.rb" "test/test_spawn.rb" "test/test_system.rb" ]

Tasks: TOP => test
(See full trace by running task with --trace)
```