[wishlist] Update grape rubygems to 1.7

We'd like to use grape 1.6.2 in debian, though grape 1.6.2 no longer has Grape::Validations::Base and rake db:migrate fails due to this.

As a workaround we can use grape 1.5.2 for now, but it'd be good if we can use grape 1.6.2.

Update: 1.7.0 may be a better option as it fixed the regression in 1.6.x versions