downgraded drone from buster to stretch, fixed travis

.drone.yml

@@ -1,11 +1,14 @@
 pipeline:
   build:
-    image: debian:buster-slim
+    image: debian:stretch-slim
     pull: true
     commands:
+      - echo "deb http://deb.debian.org/debian buster main" >> /etc/apt/sources.list.d/buster.list
+      - echo "APT::Default-Release \"stretch\";" >> /etc/apt/apt.conf.d/00default_release
       - apt-get update -q
       - apt-get install -qy build-essential cmake pkg-config
-      - apt-get install -qy libcurl4-openssl-dev libcurlpp-dev libjsoncpp-dev
+      - apt-get install -qy libcurl4-openssl-dev libjsoncpp-dev
+      - apt-get install -qy -t buster libcurlpp-dev
       - apt-get install -qy rpm file
       - mkdir -p build && cd build
       - cmake -DWITH_EXAMPLES=ON -DWITH_TESTS=ON ..

.travis.yml

@@ -9,11 +9,11 @@ addons:
       - ubuntu-toolchain-r-test
       - sourceline: 'ppa:cspensky/sda'
     packages:
-      - g++-5
+      - g++-6
       - libcurlpp-dev
       - libjsoncpp-dev
 before_install:
-  - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-5 60
+  - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-6 60
 script:
   - mkdir -p build && cd build
   - cmake -DWITH_EXAMPLES=ON -DWITH_TESTS=ON ..