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ARROW-2958: [C++] Bump Flatbuffers EP version to master to build on gcc 8.1

Verified the fix on gcc-8 on Ubuntu 18.04

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

Closes #2564 from wesm/ARROW-2958 and squashes the following commits:

fcb4419de <Wes McKinney> Bump Flatbuffers version to master to build on gcc 8.1