CI: Test with clang-7 on buster.

.drone.yml

@@ -61,20 +61,17 @@ steps:
     path: /var/cache/apt/archives
 
 - name: clang6
-  image: debian:stretch-slim
+  image: debian:buster-slim
   pull: always
   environment:
-    CXX: clang++-6.0
+    CXX: clang++
     CXXFLAGS: -pipe -O2
     LANG: en_US.utf-8
   commands:
   - rm /etc/apt/apt.conf.d/docker-clean
   - alias apt-get='rm -f /var/cache/apt/archives/lock && apt-get'
-  - echo "APT::Default-Release \"stretch\";" >> /etc/apt/apt.conf.d/00default_release
-  - echo "deb http://deb.debian.org/debian stretch-backports main" >> /etc/apt/sources.list.d/stretch.list
   - apt-get update -q
-  - apt-get install -qy -t stretch-backports clang-6.0
-  - apt-get install -qy cmake pkg-config libmagick++-dev
+  - apt-get install -qy clang cmake pkg-config libmagick++-dev
   - rm -rf build && mkdir -p build && cd build
   - cmake ..
   - cmake --build .

README.md

@@ -76,7 +76,7 @@ key](https://tastytea.de/tastytea_autosign.asc).
 #### Dependencies
 
 * C++ compiler (tested: [gcc](https://gcc.gnu.org/) 6/8/9,
-  [clang](https://llvm.org/) 6)
+  [clang](https://llvm.org/) 7)
 * [cmake](https://cmake.org/) (at least 3.6)
 * [imagemagick](https://www.imagemagick.org/) (tested: 7.0 / 6.7)
 * Optional: