Specify dependencies in qrb.gemspec

I got this when I tried to install qrb:

```
$ gem install qrb
Fetching: qrb-0.1.0.gem (100%)
Successfully installed qrb-0.1.0
Parsing documentation for qrb-0.1.0
Installing ri documentation for qrb-0.1.0
Done installing documentation for qrb after 0 seconds
1 gem installed
~ $ pry -rqrb
/Users/magnus/.rubies/ruby-2.1.0/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- citrus (LoadError)
```