makeKeysRelative(map, wd) {
    const newCoverage = {};

    Object
     .keys(map)
     .forEach(pathKey => newCoverage[path.relative(wd, pathKey)] = map[pathKey]);

    return newCoverage;
  }
Relativizes path keys so that istanbul report can be read on Windows
@param  {Object} map  coverage map generated by coverageMap
@param  {String} wd   working directory
@return {Object}      map with relativized keys