Fix uninitialized constant Earthquake::Core::YAML

This happened with ruby 2.1.1

```
/home/chatgris/.gem/ruby/2.1.1/gems/earthquake-1.0.0/lib/earthquake/core.rb:57:in `default_config': uninitialized constant Earthquake::Core::YAML (NameError)
from /home/chatgris/.gem/ruby/2.1.1/gems/earthquake-1.0.0/lib/earthquake/core.rb:80:in `load_config'
from /home/chatgris/.gem/ruby/2.1.1/gems/earthquake-1.0.0/lib/earthquake/core.rb:39:in `_init'
from /home/chatgris/.gem/ruby/2.1.1/gems/earthquake-1.0.0/lib/earthquake/core.rb:117:in `__init'
from /home/chatgris/.gem/ruby/2.1.1/gems/earthquake-1.0.0/lib/earthquake/core.rb:127:in `start'
from /home/chatgris/.gem/ruby/2.1.1/gems/earthquake-1.0.0/bin/earthquake:38:in `<top (required)>'
from /home/chatgris/.gem/ruby/2.1.1/bin/earthquake:23:in `load'
from /home/chatgris/.gem/ruby/2.1.1/bin/earthquake:23:in `<main>'
```

Signed-off-by: chatgris <jboyer@af83.com>