[travis] Use newer clang 3.7 for linux builds

ci/travis/linux/before_install.sh

@@ -1,9 +1,14 @@
 export DEBIAN_FRONTEND=noninteractive
+
+wget -O - http://llvm.org/apt/llvm-snapshot.gpg.key | sudo apt-key add -
+sudo add-apt-repository 'deb http://llvm.org/apt/precise/ llvm-toolchain-precise-3.7 main' -y
+
 sudo add-apt-repository ppa:ubuntugis/ppa -y
 sudo add-apt-repository ppa:ubuntugis/ubuntugis-unstable -y # For postgresql-9.1-postgis-2.1
 sudo add-apt-repository ppa:grass/grass-stable -y
 sudo add-apt-repository ppa:smspillaz/cmake-3.0.2 -y
 sudo add-apt-repository ppa:kedazo/doxygen-updates-precise -y # For doxygen 1.8.8
+sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
 sudo apt-get update -qq
 sudo apt-get install --force-yes --no-install-recommends --no-install-suggests \
         bison \
@@ -54,5 +59,11 @@ sudo apt-get install --force-yes --no-install-recommends --no-install-suggests \
         xfonts-scalable \
         xvfb \
         postgresql-9.1-postgis-2.1/precise # from ubuntugis-unstable, not pgdg
+
+#update clang
+sudo apt-get install --force-yes llvm-3.7 llvm-3.7-dev clang-3.7 libstdc++-4.9-dev
+export CXX="clang++-3.7" 
+export CC="clang-3.7"
+
 cmake --version
 clang --version

ci/travis/linux/install.sh

@@ -1,5 +1,7 @@
 mkdir build
 cd build
+export CXX="clang++-3.7"
+export CC="clang-3.7"
 cmake -DWITH_SERVER=ON \
       -DWITH_STAGED_PLUGINS=OFF \
       -DWITH_GRASS=ON \