Preparing for release of version 6.4.1

prepare 6.4.0 release (launchdarkly#95)

prepare 6.3.0 release (launchdarkly#94)

prepare 6.2.0 release (launchdarkly#93)

Preparing for release of version 6.1.1

Preparing for release of version 6.1.0

Preparing for release of version 6.0.4

Preparing for release of version 6.0.3

Preparing for release of version 6.0.2

Preparing for release of version 6.0.1