Thin 2.0.0.pre introduces breaking change

When using thin version 2.0.0.pre flapjack fails to start producing the following error.

```
uninitialized constant Thin::Logging
/var/lib/gems/2.1.0/gems/flapjack-0.8.5/lib/flapjack/pikelet.rb:225:in `create'
/var/lib/gems/2.1.0/gems/flapjack-0.8.5/lib/flapjack/pikelet.rb:62:in `block in create'
/var/lib/gems/2.1.0/gems/flapjack-0.8.5/lib/flapjack/pikelet.rb:60:in `each'
/var/lib/gems/2.1.0/gems/flapjack-0.8.5/lib/flapjack/pikelet.rb:60:in `create'
/var/lib/gems/2.1.0/gems/flapjack-0.8.5/lib/flapjack/coordinator.rb:132:in `block in add_pikelets'
/var/lib/gems/2.1.0/gems/flapjack-0.8.5/lib/flapjack/coordinator.rb:131:in `each_pair'
/var/lib/gems/2.1.0/gems/flapjack-0.8.5/lib/flapjack/coordinator.rb:131:in `add_pikelets'
/var/lib/gems/2.1.0/gems/flapjack-0.8.5/lib/flapjack/coordinator.rb:36:in `block in start'
/var/lib/gems/2.1.0/gems/em-synchrony-1.0.3/lib/em-synchrony.rb:38:in `block (2 levels) in synchrony'
```

This is addressed by using the thin gem version 1.6.1 and is set in the flapjack.gemspec file.