ARROW-4368: [Docs] Fix install document for Ubuntu 16.04 or earlier
ARROW-4368: [Docs] Fix install document for Ubuntu 16.04 or earlier

Author: Krisztián Szűcs <szucs.krisztian@gmail.com>
Author: Kouhei Sutou <kou@clear-code.com>

Closes #3495 from kou/doc-install-old-ubuntu and squashes the following commits:

be4e226f <Krisztián Szűcs>  fix curl command
9a7520d2 <Kouhei Sutou>  Fix install document for Ubuntu 16.04 or earlier

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