Travis: test against coveragepy 4.x

.travis.yml

@@ -5,9 +5,17 @@ env:
     - CI_TARGET=vim MAKTABA_VERSION=1.12.0
     - CI_TARGET=vim MAKTABA_VERSION=master
     - CI_TARGET=neovim MAKTABA_VERSION=master
+    - CI_TARGET=vim MAKTABA_VERSION=master COVERAGEPY="coverage>4<5"
 before_script:
   - sudo apt-get update
-  - sudo apt-get install python3-dev python-coverage
+  - sudo apt-get install python3-dev
+  - if [ -z "$COVERAGEPY" ]; then
+      sudo apt-get install python-coverage;
+    else
+      wget https://bootstrap.pypa.io/get-pip.py &&
+      sudo python3 get-pip.py &&
+      sudo pip3 install "$COVERAGEPY";
+    fi
   - if [ $CI_TARGET = vim ]; then
       sudo apt-get install vim-gnome &&
       export DISPLAY=:99.0 &&
@@ -22,6 +30,11 @@ before_script:
   - sudo dpkg -i ./vroom_0.12.0-1_all.deb
   - git clone -b ${MAKTABA_VERSION} https://github.com/google/vim-maktaba.git ../maktaba/
 script:
+  - if [ -z "$COVERAGEPY" ]; then
+      python-coverage --version;
+    else
+      coverage --version;
+    fi
   - '[ $CI_TARGET = neovim ] && VROOM_ARGS="--neovim" || VROOM_ARGS=""'
   - vroom $VROOM_ARGS --crawl ./vroom/
 matrix: