Update version to 0.6.4 and use publish task for Gradle

[oguzkocer]

Using publishPluginMavenPublicationToS3Repository Gradle task was not triggering publishConfigurePluginMarkerMavenPublicationToS3Repository which is necessary to publish the plugin marker artifacts. Instead of adding the publishConfigurePluginMarkerMavenPublicationToS3Repository to the task list, I decided to simply use the publish task, which is what we do in most repositories. I was originally concerned with publish task publishing other publications, so I wanted to use a specific task to future proof it. However, it's probably not something worth worrying about, so I think we should simplify the process.

[jkmassel]

I'm pretty sure this'll work for all the non-gradle stuff. I trust you for the gradle task 🙂