need 'require "yaml"' for Ruby 2.1.0p0

I had an error like this:

```
home/babie/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/earthquake-1.0.0/lib/earthquake/core.rb:57:in `default_config': uninitialized constant Earthquake::Core::YAML (NameError)
	from /home/babie/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/earthquake-1.0.0/lib/earthquake/core.rb:80:in `load_config'
	from /home/babie/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/earthquake-1.0.0/lib/earthquake/core.rb:39:in `_init'
	from /home/babie/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/earthquake-1.0.0/lib/earthquake/core.rb:117:in `__init'
	from /home/babie/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/earthquake-1.0.0/lib/earthquake/core.rb:127:in `start'
	from /home/babie/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/earthquake-1.0.0/bin/earthquake:38:in `<top (required)>'
	from /home/babie/.rbenv/versions/2.1.0/bin/earthquake:23:in `load'
	from /home/babie/.rbenv/versions/2.1.0/bin/earthquake:23:in `<main>'
```

So I added this workaround and it work fine.