Deploy to latest 3.6

[Giovanni1085]

In order to deploy to the latest package versions on 3.6 I have done the following:

• add nbformat to requirements.txt
• edit merge.py as follows:

  from nbformat import read as nb_read
  from nbformat import writes as nb_write
  ... 
  nb = nb_read(f, as_version=4)
  ... 
  result = nb_write(merged)

• edit .travis.yml to:

• gem install liquid -v 4.0.0
• gem install jekyll -v 3.8.4

Hope this helps.