Update grape to 0.16.1

grape 0.13.0, which we're currently on, depends on the rack-mount gem, which was discontinued 5 years ago. It has been removed as dependency from grape as of 0.16.1: https://github.com/ruby-grape/grape/blob/master/CHANGELOG.md#0161-432016

GitLab depending on rack-mount is blocking the creation of a native Fedora package per https://gitlab.com/gitlab-org/gitlab-ce/issues/14043#note_11654776.

@connorshea Would you like to have a look at this? You've been busy slashing gems left and right already.