CHANGELOG: prepare for 0.5.1 release

v0.5.0 release!

dep: Sharpen `dep prune` warning message

Merge pull request golang#1559 from sdboyer/v040-release-prep

v0.4.0 release prep

Merge pull request golang#1284 from shawnps/patch-1

fix typo

Prep CHANGELOG for release

Merge pull request golang#489 from sdboyer/new-ensure

Implement new spec for `dep ensure`

Merge pull request golang#781 from ibrasho-forks/fix-symlink-cloning-…

…on-windows

internal/fs: don't clone symlinks on windows

Remove some old TODOs that no longer apply

Merge meta-PR to stabilize Gopkg metadata files