prep for 1.6.0 release (PowerShell#628)

* prep for 1.6.0 release.

* add numbers

Merge pull request PowerShell#578 from PowerShell/rkeithhill/prep-rel…

…ease

Prep for 1.5.1 release.

Bump version to 1.5.0, update CHANGELOG.md

Bump version to 1.4.1, update CHANGELOG.md

Bump version to 1.4.0, update CHANGELOG.md

Bump version to 1.3.2, update CHANGELOG.md

Bump version to 1.3.1, update CHANGELOG.md

Bump version to v1.3.0, update CHANGELOG.md

Bump version to 1.2.1, update CHANGELOG.md

Bump version to 1.2.0, update CHANGELOG.md