Fix changelog for v1.9.0

Prepare for v1.8.0

Prepare to release v1.7.0 (uber-go#502)

Update the changelog for the 1.7.0 release.

Update changelog for 1.6.0 release (uber-go#498)

Cutting a release so we can reduce the noise in stacktraces.

Update changelog for 1.5.0 release (uber-go#481)

Two features and a bugfix are enough to warrant a minor release.