Specifying version of cppcheck fails on MacOS and Linux #33

Attempting to set cppcheck: 2.6 on MacOS or Linux fails to install a package.

However, it succeeds with no problem on Windows.

lefticus/json2cpp#7

(invalid syntax for brew?)
MacOS failure: https://github.com/lefticus/json2cpp/runs/5147554453?check_suite_focus=true#step:6:76

(ubuntu 20.04 package manager doesn't know cppcheck version 2.6)
Ubuntu Failure: https://github.com/lefticus/json2cpp/runs/5147554297?check_suite_focus=true#step:6:111