Use Lua 5.1 when running "make coverage-report" on GitLab CI
Use Lua 5.1 when running "make coverage-report" on GitLab CI

Since the "cdev-debian" Docker image used for the "pages" job
currently only installs luacov for Lua 5.1.

craigbarnes/lua-gumbo,craigbarnes/lua-gumbo,craigbarnes/lua-gumbo