Migrate to Kotlin Latest Version of Kotlin

[dlurton]

The big feature we need here is the explicit API mode for library authors, which makes missing visibility specifiers that are exposed to the public API either a warning or an error. This is has been a big pain point for us since it is difficult to control the API surface area that we expose.

Enabling this feature will be a separate task.

[RCHowell]

Explicit library mode is in 1.4 which PIG is already on. Closing this, but keeping #64 open