Changing clang tp 3.9 as 3.7 no longer works on Travis

ci/install-linux.sh

@@ -41,7 +41,7 @@ if [ "${TRAVIS_SUDO}" = "true" ]; then
     echo "deb [arch=amd64] http://storage.googleapis.com/bazel-apt stable jdk1.8" | \
         sudo tee /etc/apt/sources.list.d/bazel.list
     curl https://bazel.build/bazel-release.pub.gpg | sudo apt-key add -
-    sudo apt-get update && sudo apt-get install -y bazel gcc-4.9 g++-4.9 clang-3.7
+    sudo apt-get update && sudo apt-get install -y bazel gcc-4.9 g++-4.9 clang-3.9
 elif [ "${CXX}" = "clang++" ]; then
     # Use ccache, assuming $HOME/bin is in the path, which is true in the Travis build environment.
     ln -sf /usr/bin/ccache $HOME/bin/${CXX};