Merge pull request #179 from tkf/remove-known-failures-tag

Remove tags for known failures due to the import hack
Merge pull request #178 from tkf/no-duplicate-module

Add a test to make sure that the import hack works
Merge pull request #182 from tkf/Fix-Completion.follow_definition

Fix Completion.follow_definition
remove __all__ from api, because not needed, fixes #180
Remove old version of FS cache automatically
Separate directory for each Python version
Compare version using "!="

Cache is not forward compatible.