pin versions for module installations

For each module to install, this change will check for a corresponding environment variable that will hold the max version number allowed to be installed.

for `DDG::SpiceBundle::OpenSourceDuckDuckGo`, it would check for `DDG_SpiceBundle_OpenSourceDuckDuckGo`.

This value will hold the max version number to install. It will only install this version if it is greater than the one currently installed.

It is possible that we could specify this on the command line, like `duckpan module@version` but this is slightly different that specifying a version number to install. Or is it?

@yegg @nilnilnil 
add that this is a URL