Bump airlift and tpch versions to non snapshot versions
[maven-release-plugin] prepare release 0.87
[maven-release-plugin] prepare for next development iteration
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());
Minor improvements to ImmutableCollectors

- Rename methods to toImmutableList and toImmutableSet
- Simplify method signatures
- Remove unnecessary type specification for lambda args
- Declare set collector as UNORDERED to give stream implementation
  more room for optimization
Implement ImmutableCollectors.toImmutableMultiset
Deprecate IterableTransformer
Enable modernizer plugin

For now, start with the stock set of violations and only warn.
Change Travis config to use Java 8
Reduce memory footprint in Travis
Don't fork compiler

This speeds up compiling by ~25% and helps prevent Travis from running
out of memory