Upgrade llvm-dev and boot kcov

Need clang-dev, not llvm-dev because cargo-cov looks for libprofile
in a clang installation directory.

ci/docker-rust/Dockerfile

@@ -6,28 +6,19 @@ RUN set -x && \
     apt update && \
     apt-get install apt-transport-https && \
     echo deb https://apt.buildkite.com/buildkite-agent stable main > /etc/apt/sources.list.d/buildkite-agent.list && \
-    echo deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-6.0 main > /etc/apt/sources.list.d/llvm.list && \
+    echo deb http://apt.llvm.org/stretch/ llvm-toolchain-stretch-7 main > /etc/apt/sources.list.d/llvm.list && \
     apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 32A37959C2FA5C3C99EFBC32A79206696452D198 && \
     wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add - && \
     apt update && \
     apt install -y \
       buildkite-agent \
       cmake \
-      llvm-6.0 \
+      libclang-common-7-dev \
       rsync \
       sudo \
       && \
     rustup component add rustfmt-preview && \
     rustup component add clippy-preview && \
-    apt install -y \
-      binutils-dev \
-      libcurl4-openssl-dev \
-      zlib1g-dev \
-      libdw-dev \
-      libiberty-dev \
-      && \
-    cargo install cargo-kcov && \
-    cargo kcov --print-install-kcov-sh | VERBOSE=1 bash && \
     rm -rf /var/lib/apt/lists/* && \
     rustc --version && \
     cargo --version