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ARROW-1461: [C++] Disable builds using LLVM apt repo until installation issues resolved

It looks like llvm.org made some changes to their apt repository in the last 48 hours and this has been causing our builds to fail. We can re-enable builds using .deb packages from apt.llvm.org at a later time

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

Closes #1038 from wesm/fix-trusty-toolchain and squashes the following commits:

36ee952b [Wes McKinney] Disable clang until LLVM apt stabilizes
ff26ce6e [Wes McKinney] Update LLVM apt repo URL