Auto merge of #18955 - servo:jdm-patch-4, r=emilio

Set up llvm for osmesa.

This fixes #18954.

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.travis.yml

@@ -12,6 +12,12 @@ matrix:
       cache: false
     - sudo: 9000
       dist: trusty
+      before_install:
+        - sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
+        - sudo add-apt-repository 'deb http://apt.llvm.org/precise/ llvm-toolchain-precise-3.9 main' -y
+        - sudo apt-get update -q
+        - sudo apt-get install clang-3.9 llvm-3.9 llvm-3.9-runtime -y
+        - export LLVM_CONFIG=/usr/lib/llvm-3.9/bin/llvm-config
       script:
          - ./mach build -d --verbose
          - ./mach test-compiletest