elpy-config now points out outdated Python packages.

Outdated packages (currently only Rope and Jedi) are shown with
their newer versions and the option to upgrade them.

Fixes #297.
Ignore syntax error bug in Jedi.

This fixes #314.
Add elpy-version command again.

Fixes #322.
Ignore import errors from Jedi.

Fixes #337.
elpy-config warns about ~/.local/bin not in PATH.

The directory is used by programs installed by pip install --user,
which can cause confusion if they are not found even when they are
installed.

Fixes #324.
Sort completion results.

Elpy now sorts completion results in a more or less intuitive manner.
Normal, public symbols come first, followed by dunder symbols like
__init__, and finally private, single-underscore symbols. Additionally,
the completion results are case-insensitive.

Fixes #321.