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ARROW-3027: [Ruby] Stop "git tag" by "rake release"

The "release" task defined by Bundler runs "git tag" but it's not
needed for us.

We can release Ruby bindings by "rake release" in ruby/red-arrow/ and ruby/red-arrow-gpu/.
It should be documented at Confluence.

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

Closes #2408 from kou/ruby-stop-tagging and squashes the following commits:

49388a21 <Kouhei Sutou>  Stop "git tag" by "rake release"