Psych 4: Psych::BadAlias exception when unserializing a config/s3.yml

[yui-har]

When the plugin tries to read a YAML file like the one below, the exception Psych::BadAlias is raised.

s3_default: &default
  bucket: bucket-name
  folder: folder/files
  thumb_folder: folder/thumb

development:
  <<: *default

Psych 4.0 uses safe_load by default (see ruby/psych#487). Due to this change, config/s3.yml may raise Psych::BadAlias exception.
This bug can be resolved by using unsafe_load with Psych 4.0.

[yoshiokaCB]

Thank you for PR.