Context: everybody who fresh-installed anaconda-mode since December 25h (jedi 0.18.0 release) gets broken installation (see #401 )

At the moment, anaconda-mode installs needed python modules with loose requirement, for example jedi is installed with jedi>=0.13.0. Either exact tested version should be specified, or range with two-side restrictions (say jedi>=0.13.0,<0.18.0).

Preferably, some customizable variable could let one patch this requirement if necessary (anaconda-jedi-version…)

code-wise it's about constructing missing_dependencies variable.