added missing player file to gemspec

Added `lib/airstream/player.rb` to gemspec file, because otherwise:

```
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- airstream/player.rb (LoadError)
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /var/lib/gems/1.9.1/gems/airstream-0.4.2/lib/airstream.rb:17:in `<top (required)>'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
	from /var/lib/gems/1.9.1/gems/airstream-0.4.2/bin/airstream:6:in `<top (required)>'
	from /usr/local/bin/airstream:23:in `load'
	from /usr/local/bin/airstream:23:in `<main>'
```



BTW. The latest version on rubygems.org is still 0.3.7.  Did you push 0.4.X?