Fix failing specs for ruby 2.0.0

Specs fail in ruby 2.0:
```
Failures:                                                                     
                                                                              
  1) DeepStruct.from_file with Yaml file reads the file                       
     Failure/Error: DeepStruct.from_file(fixture_file_path("data.yaml"))      
     NameError:                                                               
       uninitialized constant YAML                                            
     # ./lib/deep_struct/file_reader.rb:36:in `read_yaml_file'                
     # ./lib/deep_struct/file_reader.rb:11:in `data'                          
     # ./lib/deep_struct.rb:23:in `from_file'                                 
     # ./spec/deep_struct_spec.rb:71:in `block (3 levels) in <top (required)>'
     # ./spec/deep_struct_spec.rb:76:in `block (3 levels) in <top (required)>'
                                                                              
  2) DeepStruct.from_file with Yaml file and short extension reads the file   
     Failure/Error: DeepStruct.from_file(fixture_file_path("data.yml"))       
     NameError:                                                               
       uninitialized constant YAML                                            
     # ./lib/deep_struct/file_reader.rb:36:in `read_yaml_file'                
     # ./lib/deep_struct/file_reader.rb:11:in `data'                          
     # ./lib/deep_struct.rb:23:in `from_file'                                 
     # ./spec/deep_struct_spec.rb:82:in `block (3 levels) in <top (required)>'
     # ./spec/deep_struct_spec.rb:87:in `block (3 levels) in <top (required)>'
```

Apparently a fix for this is simply to require `yaml` library.