[ci] cleanup CI installation (#1806)

.ci/dockers/ubuntu-14.04/Dockerfile

@@ -25,12 +25,14 @@ RUN apt-get update \
  && rm -rf /var/lib/apt/lists/*
 
 # Install clang 6.0
-RUN add-apt-repository ppa:ubuntu-toolchain-r/test \
+RUN add-apt-repository ppa:ubuntu-toolchain-r/test -y \
  && cd /tmp \
  && wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add - \
- && apt-add-repository "deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-6.0 main" \
+ && add-apt-repository "deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-6.0 main" -y \
  && apt-get update \
- && apt-get install -y --no-install-recommends clang-6.0 
+ && apt-get install -y --no-install-recommends \
+    clang-6.0 \
+ && rm -rf /var/lib/apt/lists/*
 
 # Install CMake
 RUN curl -sL https://cmake.org/files/v3.12/cmake-3.12.0-Linux-x86_64.sh -o cmake.sh \

.ci/setup.sh

@@ -29,7 +29,7 @@ else  # Linux
         sudo apt-get update
         sudo update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-6.0 100
         sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-6.0 100
-        sudo apt-get install libomp-dev
+        sudo apt-get install --no-install-recommends -y libomp-dev
     fi
     if [[ $TASK == "mpi" ]]; then
         sudo apt-get update