Revert "[travis] Use newer clang 3.7 for linux builds"

This reverts commit 54cf64bcdc6dc362fe8ca75c019ec82c7c240af9.

Seems like the recent clang 3.7.1 release is causing build
issues with the sip bindings. Revisit at some later stage, but
for now just get Travis building correctly again.

ci/travis/linux/before_install.sh

@@ -1,13 +1,9 @@
 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: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 \
@@ -65,6 +61,3 @@ sudo apt-get install --force-yes --no-install-recommends --no-install-suggests \
         postgresql-9.1-postgis-2.1/precise # from ubuntugis-unstable, not pgdg
 
 sudo -H pip install autopep8 # TODO when switching to trusty or above: replace python-pip with python-autopep8
-
-#update clang
-sudo apt-get install --force-yes llvm-3.7 llvm-3.7-dev clang-3.7 libstdc++-4.9-dev

ci/travis/linux/install.sh

@@ -1,12 +1,6 @@
 mkdir build
 cd build
 
-export CXX="clang++-3.7"
-export CC="clang-3.7"
-
-cmake --version
-${CC} --version
-
 cmake -DWITH_SERVER=ON \
       -DWITH_STAGED_PLUGINS=ON \
       -DWITH_GRASS=ON \