added fix for PR coverage affecting master coverage

This fix relies on the new changes added to CrowCpp/cpp-coveralls in 8b427c04a691867bb29a0acfc2d948a690476d79

.drone.yml

@@ -17,6 +17,7 @@ steps:
   - export DEBIAN_FRONTEND=noninteractive
   - export TRAVIS_BRANCH=$DRONE_BRANCH
   - export TRAVIS_JOB_ID=$DRONE_BUILD_NUMBER
+  - export COVERALLS_PULL_REQUEST=$DRONE_PULL_REQUEST
   - apt-get -y update
   - apt-get -y install libboost-all-dev doxygen mkdocs graphviz zlib1g-dev gcc clang make cmake python3 python3-pip git openssl libssl-dev
   - pip3 install mkdocs-material pyyaml --no-input