function resolveVersion(spec, installed) {
    //try to get a version if its explicit
    var version = semver.clean(spec);
    //support version ranges
    if(semver.validRange(spec)){
      version = semver.maxSatisfying(installed,spec);
    }
    done(null,version);
  }
Get a download URL for the version
@param {string} version
@return {string}