move maven-protoc-plugin to maven central

[saturnism]

any reason why some of the components, like maven-protoc-plugin, are in bintray? can this be moved to maven central instead?

[ejona86]

As you may have seen in sergei-ivanov/maven-protoc-plugin#11, there were details that needed to be worked out. But as of two days ago it is now available on Maven Central. I created #1431 to update the docs.