travis: add ubuntu-toolchain-r-test

LLVM 6.0 requires libstc++4.9, which is not available in main Travis
repository.

v2: LLVM 6.0 requires libstdc+4.9, rather than GCC 4.9 (Jan Vesely)

Fixes: fd1121e839 ("amd: remove support for LLVM 5.0")
CC: Marek Olšák <marek.olsak@amd.com>
CC: Emil Velikov <emil.velikov@collabora.com>
CC: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

.travis.yml

@@ -137,6 +137,8 @@ matrix:
         apt:
           sources:
             - llvm-toolchain-trusty-6.0
+            # llvm-6 depends on gcc-4.9 which is not in main repo
+            - ubuntu-toolchain-r-test
           packages:
             # LLVM packaging is broken and misses these dependencies
             - libedit-dev
@@ -378,6 +380,8 @@ matrix:
         apt:
           sources:
             - llvm-toolchain-trusty-6.0
+            # llvm-6 depends on gcc-4.9 which is not in main repo
+            - ubuntu-toolchain-r-test
           packages:
             # LLVM packaging is broken and misses these dependencies
             - libedit-dev