update grape to 0.3.1 for ruby 2.0 support.

add grape-entity, that is split from grape 0.3.0.
remove usage of deprecated error_format, that is removed in grape 0.2.3.
Merge pull request #3095 from fdiary/update-grape-for-ruby2.0

update grape to 0.3.1 for ruby 2.0 support.