Allow to install the minimum dependencies #96
Description
The HoloViz packages declare their Python runtime dependencies in the install_requires parameter of their setup.py file. In most cases these dependencies are pinned, e.g. bokeh >= 1.1. Pins are usually set at a point in time when they're known to be valid. For instance, the package relies on a new API made available in Bokeh 1.1, so the pin has to be bokeh >= 1.1 to prevent the package to be installed with older versions of Bokeh. However, these minimum pins are only tested for a short while. Each new release of the pinned dependency (e.g. Bokeh 1.2, 2.0, 2.1, etc.) makes the pin more likely to be invalid, as it might well be that the package relies on new API that would have required a minimum version bump, which could easily be missed. As the dependencies installed part of a test suite are generally the latest one available (or close to that), there's simply no way to detect this sort of issue.
A parameter could be added to the develop_install command, that when set would force the installation of the minimum declared dependency versions (e.g. bokeh==1.1).
Some packages don't pin all their dependencies:
https://github.com/holoviz/hvplot/blob/master/setup.py#L34
I believe that in practice it is pretty unlikely for a package to work with any version of one of its dependencies. I would suggest that doit develop_install --new-flag should raise an error when it finds an unpinned runtime dependency.