Feature/fix issue 873

Add LRU cache to RequirementsFinder._get_names

Lookup real path of virtualenv

This ensures we don't get confused and incorrectly classify third party
libraries as first party libraries.

Fixes #876.

This should speed up the isort invocations significantly, like what
https://github.com/timothycrosley/isort/pull/856 intended to do, but somehow
missed the mark.

The `_get_names` method of RequirementsFinder seem to be the biggest culprit
and not `_get_files_from_dir`. I think I confused my previous benchmark results
due to `PipfileFinder` and `RequirementsFinder` being enabled in the same
change, so I may potentially have swapped around the benchmark results ‍♂️

Attempt to add mac testing

This also failed for me with "future" being installed.

Not sure if it makes sense to keep it in the first place, since isort
appears to require py34 now.

Ref: https://github.com/timothycrosley/isort/issues/479#issuecomment-316235116

Restore `chdir` after tests.

This is relevant for when pytest-cov would display coverage in the end,
and would use absolute paths due to this.

Fix whitespace in warning message

Feature/fix windows support