Travis: build with clang, so that we can use its sanitizers.

This also disables coverage, because llvm-cov 3.9 on Travis is
incompatible with genhtml 1.10. Sigh.

.travis/build-debian.sh

@@ -4,8 +4,7 @@ if echo $TRAVIS_TAG | grep ^v; then BUILD_TYPE=RelWithDebInfo; else BUILD_TYPE=D
 
 mkdir build
 cd build
-cmake .. -DCMAKE_C_COMPILER=gcc-5 -DCMAKE_CXX_COMPILER=g++-5 \
+cmake .. -DCMAKE_C_COMPILER=clang-3.9 -DCMAKE_CXX_COMPILER=clang++-3.9 \
   -DCMAKE_BUILD_TYPE=$BUILD_TYPE \
-  -DENABLE_COVERAGE=ON \
   -DENABLE_SANITIZERS=ON
 make VERBOSE=1

.travis/install-debian.sh

@@ -1,8 +1,10 @@
 #!/bin/sh -xe
 
+wget -O - http://apt.llvm.org/llvm-snapshot.gpg.key|sudo apt-key add -
 sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
+sudo add-apt-repository -y 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-3.9 main'
 sudo apt-get update -qq
 sudo apt-get install -q -y \
   cmake cmake-data libpng12-dev zlib1g-dev libjson0-dev libfontconfig1-dev \
   libgtkmm-2.4-dev libpangomm-1.4-dev libcairo2-dev libgl1-mesa-dev libglu-dev \
-  libfreetype6-dev dpkg-dev gcc-5 g++-5 lcov
+  libfreetype6-dev dpkg-dev clang-3.9 clang++-3.9 lcov