Fix Python dependency versioning

[mrecachinas]

We removed the versions to fix #47, but it seems this is breaking fresh installations of Jupyter-SigPlot in Python 2 (Python 3 works).

@RinconResearch @sterre what versions are you and your customers able to use? My environment is very flexible, so I can use the latest version, but one of my customers is constrained by Jupyter 5.2.2 and ipywidgets==6.0.2 (still need to fix this, see #40.

cc @jrmims

[sterre]

For my own projects, I'm trying to stick with the December 2018 Anaconda release (whichever Python 2/3 and Jupyter version is in that).

We're thinking of making a Docker container for lab users, but haven't gotten around to it yet. I'll likely use the same versions there (Dec 2018).

I'll need to check with @RinconResearch about what he's seeing as the "native" lab installs, but at present, we're delivering this as a proof of concept, so we don't have a hard version dependency. I hope to get a handful more users drafted in the next couple of weeks. They may have more concrete version requirements.