Minor fix to "make coverage-report" target
Minor fix to "make coverage-report" target

It was running "make check-lua51" and then "lua runtests.lua", which
means it was running the Lua tests twice. It now prepares the build
with "make build-lua51" and leaves runtests.lua to be run manually
with the correct flags ("-lluacov").

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