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
Load java.* classes from parent classloader
Convert Accumulator to take a BlockBuilder
Implement arbitrary aggregation function
Fix an issue when using row_number() and limit() together