CI: Disable GCC 6 test.

I don't know how to fix this error:

+ apt-get install -qq -t stretch-backports cmake
E: Unable to correct problems, you have held broken packages.

.drone.yml

@@ -19,22 +19,43 @@ steps:
     source: https://schlomp.space/attachments/f40d4b6e-ab9f-4cb7-a83e-cb1d8490e9ee
     destination: identiconpp_stretch.deb
 
-- name: GCC 6 / clang 7
-  image: debian:stretch-slim
+# - name: GCC 6
+#   image: debian:stretch-slim
+#   pull: always
+#   environment:
+#     CXX: g++-6
+#     CXXFLAGS: -pipe -O2
+#     DEBIAN_FRONTEND: noninteractive
+#     LANG: C.utf8
+#   commands:
+#   - rm /etc/apt/apt.conf.d/docker-clean
+#   - 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 -qq -t stretch-backports cmake
+#   - apt-get install -qq g++-6 pkg-config
+#   - apt-get install -qq libcrypto++-dev libmagick++-dev libxdg-basedir-dev
+#   - dpkg -i identiconpp_stretch.deb
+#   - rm -rf build && mkdir -p build && cd build
+#   - cmake ..
+#   - cmake --build .
+#   - make install
+#   volumes:
+#   - name: debian-package-cache
+#     path: /var/cache/apt/archives
+
+- name: GCC 8 / clang 7
+  image: debian:buster-slim
   pull: always
   environment:
-    CXX: g++-6
+    CXX: g++-8
     CXXFLAGS: -pipe -O2
     DEBIAN_FRONTEND: noninteractive
     LANG: C.utf8
   commands:
   - rm /etc/apt/apt.conf.d/docker-clean
   - apt-get update -q
-  - 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 -qq -t stretch-backports cmake
-  - apt-get install -qq g++-6 clang-7 pkg-config
+  - apt-get install -qq build-essential clang pkg-config cmake
   - apt-get install -qq libcrypto++-dev libmagick++-dev libxdg-basedir-dev
   - dpkg -i identiconpp_stretch.deb
   - rm -rf build && mkdir -p build && cd build

README.md

@@ -133,7 +133,7 @@ Up to and including 0.6.2, the packages were generated on Ubuntu 16.04 64 bit.
 
 #### Dependencies
 
-* C++ compiler (tested: [gcc](https://gcc.gnu.org/) 6/9,
+* C++ compiler (tested: [gcc](https://gcc.gnu.org/) 8/9,
   [clang](https://llvm.org/) 7)
 * [cmake](https://cmake.org/) (at least 3.10)
 * [crypto++](https://cryptopp.com) (tested: 7.0 / 5.6)