Refactor installation step in the cmakecheck job

Author: harrisonmutai-arm

.travis.yml

@@ -346,6 +346,7 @@ matrix:
       stage: "Extended"
       name: "cmakecheck"
       env: NAME=cmakecheck
+      python: 3.7
       install:
         # Install python modules
         - python -m pip install --upgrade pip==18.1