Update dcmjs.js based on commontk/dcmjs@3995ab2

Note that following the introduction of .mem file, it is now required
to set the 'memoryInitializerPrefixURL'. See kripken/emscripten#2537
Merge branch 'update-dcmjs' into gh-pages

* update-dcmjs:
  Update dcmjs.js based on commontk/dcmjs@3995ab2