delete full_name from gemspec

```
➜ bundle update xmpp4r
Updating git://github.com/xmpp4r/xmpp4r.git
There was a NoMethodError while loading xmpp4r.gemspec:
undefined method `full_name=' for #<Gem::Specification:0x3fecd1b09bec -> from
  /Users/username/.rvm/gems/ruby-2.0.0-p481@myapp/bundler/gems/xmpp4r-dae0f84d116c/xmpp4r.gemspec:236:in `block in <main>'
```