Squashed:
* fix: CheckHealth doesn't work in neovim
CheckHealth calls display_debug_info().
However, error occured when loading jedi_vim.jedi.Script('')._evaluator.sys_path,
because Evaluator.sys_path is unavailable variable.
* handle the both the old and new API
Evaluator.project.sys_path is temporary fix, because Evaluator.sys_path is correct API.
So, we handle the both the old and new API in preparation for merging virtualenv branch.
This allows vimlparser to parse the vim file again, and it is more
convenient to have it in a real Python file anyway.
Small refactoring and minor text changes included.
