remove references to non-existant files from gemspec

Just started to check this out, sounds ideal for my use case. 

While this isn't in rubygems.org it'd be nice to be able to build with something like https://github.com/rdp/specific_install

````shell
$ gem build etcd-chef.gemspec 
ERROR:  While executing gem ... (Gem::InvalidSpecificationException)
    ["LICENSE", "ROADMAP.md", "CHANGELOG.md"] are not files
```