message: build(build): limit ImageIO build parallelism
  To avoid intermittent issues with libcxx.a as described here:
  https://groups.google.com/forum/#!topic/emscripten-discuss/llX0HtHyxN8
diff: @@ -73,7 +73,7 @@ try {
  if (err.code != 'EEXIST') throw err
  }
  try {
  - fs.mkdirSync(path.join('dist', 'itkWebWorkers'))
  + fs.mkdirSync(path.join('dist', 'WebWorkers'))
  } catch(err) {
  if (err.code != 'EEXIST') throw err
  }
  @@ -140,7 +140,7 @@ const browserifyBuild = ramda.curry(function (outputDir, es6File, callback) {
  })
  const browserifyWebWorkerBuildParallel = function (callback) {
  const es6Files = glob.sync(path.join('src', 'WebWorkers', '*.js'))
  - const outputDir = path.join('dist', 'itkWebWorkers')
  + const outputDir = path.join('dist', 'WebWorkers')
  builder = browserifyBuild(outputDir)
  result = asyncMod.map(es6Files, builder)
  callback(null, result)
  