Use version 6.1.1 of checkstyle library

The version used by the checkstyle maven plugin doesn't support Java 8,
so override it manually until the plugin is fixed
Add utility for bridging Guava builders with Java 8 streams

This allows us to do the following and get an ImmutableList or ImmutableSet:

List<Integer> list = collection.stream()
    .map(...)
    .filter(...)
    .collect(ImmutableCollectors.immutableListCollector());

Set<Integer> set = collection.stream()
    .map(...)
    .filter(...)
    .collect(ImmutableCollectors.immutableSetCollector());