Restore evaluating environment markers in WorkingSet

Correctly deal with parsed requirements that include extras as a
marker inside WorkingSet that are populated from METADATA inside
wheels, like we get from pip>=7.

This partially reverts commit 04d10ff0.

Closes: #523

Drop unused safe_repr function

When _assertIn was removed from test_resources, the safe_repr
function was not dropped, leaving it with no callers, so drop it.

--HG--
branch : distribute

Update vendored packaging to 16.6

Don't test the bootstrap script. It fails when deploys are happening, which is common when releases are being made.

Logic in upload_docs will only trigger docs build if no docs dir was specified, so explicitly build during release. Fixes #524.