Peg version of jupyter

[riastradh-probcomp]

Currently we have two dockerfiles: one (ubuntu1604) that uses only apt packages from Ubuntu 16.04 and our probcomp apt repository, and another (ubuntu1604-jupyter) that installs everything except jupyter from Ubuntu 16.04 apt packages and installs jupyter and ipython<6 from pypi with pip.

• The version of ipython notebooks in Ubuntu 16.04 is too old for our needs -- we require jupyter notebooks for something or other.
• We should pick a particular version of jupyter that works for us to limit bitrot of the stack.
• ipython>=6 requires python3; all our software uses python2.
• It would be nice to get security updates for jupyter.
• It would be nice not to expose all our users to vulnerability from compromised pypi, which is neither curated or audited.

We should commit to one option.

[axch]

Under the circumstances, I think Jupyter from pip is the only option we have. Pegging a specific version (presumably the latest that exists <6) is, I'm afraid, the best we can do.