Create the symlink inside the if

The problem having it outside of the if is that the untar resource
doesn't exist if maven is already installed. I justify moving it inside
becasue if the symlink doesn't exist, then it is likely that
$::maven_version will not be the same as $version, and will install the
version specified, thus creating the new symlink.
Merge pull request #24 from Oakie3CR/fix_link_dependency

Create the symlink inside the if