CI: Fix GCC version in bionic.

.drone.yml

@@ -163,7 +163,7 @@ steps:
   image: ubuntu:bionic
   pull: always
   environment:
-    CXX: g++-7
+    CXX: g++-8
     CXXFLAGS: -pipe -O2
     DEBIAN_FRONTEND: noninteractive
     LANG: C.UTF-8
@@ -171,7 +171,7 @@ steps:
   - rm /etc/apt/apt.conf.d/docker-clean
   - alias apt-get='rm -f /var/cache/apt/archives/lock && apt-get'
   - apt-get update -q
-  - apt-get install -qq build-essential clang locales lsb-release
+  - apt-get install -qq g++-8 build-essential clang locales lsb-release
   - apt-get install -qq catch libboost-program-options-dev libboost-locale-dev libboost-regex-dev gettext libarchive-dev libfmt-dev asciidoc
   - sh cmake_installer.sh --skip-license --exclude-subdir --prefix=/usr/local
   - rm -rf build && mkdir -p build && cd build