Remove duplicate keys

Remove warnings in ruby 2.2+.

```
/home/badlamer/.rvm/gems/ruby-2.2.0/gems/rghost-0.9.5/lib/rghost/constants.rb:68: warning: duplicated key at line 82 ignored: :pxlmono
/home/badlamer/.rvm/gems/ruby-2.2.0/gems/rghost-0.9.5/lib/rghost/constants.rb:69: warning: duplicated key at line 83 ignored: :pxlcolor
```