function hashPrerelease(s) {
  if (_.isString(s[0])) {
    return (_.indexOf(PRERELEASE_CHANNELS, s[0]) + 1) * PRERELEASE_CHANNEL_MAGINITUDE + (s[1] || 0);
  } else {
    return s[0];
  }
}
Hash a prerelease
@param  {String} s [description]
@return {String}   [description]