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ARROW-5339: [C++] Add jemalloc URL to thirdparty/versions.txt so download_dependencies.sh gets it

I confirmed that I'm able to build the project with jemalloc with no networking disabled with this change

Author: Wes McKinney <wesm+git@apache.org>

Closes #4533 from wesm/ARROW-5339 and squashes the following commits:

6b627ea1d <Wes McKinney> Add jemalloc URL to thirdparty/versions.txt