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ARROW-1135: [C++] Use clang 4.0 in one of the Linux builds

This will help us keep the Linux build clean on latest clang

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #779 from wesm/ARROW-1135 and squashes the following commits:

ca8ba32b [Wes McKinney] Fix dependency graph problems. Disable sign-conversion warnings
e5d3efb1 [Wes McKinney] Upgrade Flatbuffers to 1.7.1, because build fails in 1.6.0 on clang 4.0
f186b926 [Wes McKinney] Add clang installer script
24bb6c63 [Wes McKinney] Use clang 4.0 in one of the Linux builds