Move VERSION to clipboard/version.rb

I don't think code in lib/ should be reaching outside of lib/ to grab version number. I think you should be able to just require lib/clipboard.rb in isolation and everything should just work. 

Reason I say this is because I've been trying to make a self-contained .jar app in JRuby that uses unpacked gem dependencies so I've been including all the lib/* code for all the gems. Clipboard has been throwing me errors because it can't find the VERSION file. 

I looked at the Sinatra gem for reference and they do a similar pattern, and also this post here: http://timelessrepo.com/making-ruby-gems