Use sudo build and manually install clang-3.7

Travis have removed llvm-toolchain-precise-3.7 from their whitelist
of sources for the "apt:" option, so we have to use a sudo build and
add it as a source ourselves.

Author: Owen Jones

.travis.yml

@@ -149,22 +149,24 @@ jobs:
     # Ubuntu Linux with glibc using clang++-3.7, no-debug mode
     - stage: Test different OS/CXX/Flags
       os: linux
-      sudo: false
+      sudo: true
       compiler: clang
       cache: ccache
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-3.7
           packages:
             - libwww-perl
-            - clang-3.7
             - g++-5
             - libstdc++-5-dev
             - libubsan0
             - parallel
       before_install:
+        - curl -sSL "http://apt.llvm.org/llvm-snapshot.gpg.key" | sudo -E apt-key add -
+        - echo "deb http://apt.llvm.org/precise/ llvm-toolchain-precise-3.7 main" | sudo tee -a /etc/apt/sources.list > /dev/null
+        - sudo apt-get update --option Acquire::Retries=100 --option Acquire::http::Timeout="60"
+        - sudo -E apt-get -yq --no-install-suggests --no-install-recommends --force-yes install clang-3.7
         - mkdir bin
         - ln -s /usr/bin/gcc-5 bin/gcc
         - ln -s /usr/bin/c++-5 bin/g++
@@ -178,21 +180,23 @@ jobs:
     # Ubuntu Linux with glibc using clang++-3.7, debug mode, disable USE_DSTRING
     - stage: Test different OS/CXX/Flags
       os: linux
-      sudo: false
+      sudo: true
       compiler: clang
       cache: ccache
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-3.7
           packages:
             - libwww-perl
-            - clang-3.7
             - g++-5
             - libstdc++-5-dev
             - libubsan0
       before_install:
+        - curl -sSL "http://apt.llvm.org/llvm-snapshot.gpg.key" | sudo -E apt-key add -
+        - echo "deb http://apt.llvm.org/precise/ llvm-toolchain-precise-3.7 main" | sudo tee -a /etc/apt/sources.list > /dev/null
+        - sudo apt-get update --option Acquire::Retries=100 --option Acquire::http::Timeout="60"
+        - sudo -E apt-get -yq --no-install-suggests --no-install-recommends --force-yes install clang-3.7
         - mkdir bin
         - ln -s /usr/bin/gcc-5 bin/gcc
         - ln -s /usr/bin/g++-5 bin/g++