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ARROW-3431: [GLib] Include Gemfile to archive

It's helpful to setup test environment.

Author: Kouhei Sutou <kou@clear-code.com>

Closes #2702 from kou/c-glib-add-missing-gemfile-to-archive and squashes the following commits:

5c97dad1b <Kouhei Sutou>  Include Gemfile to archive