Improved h2o-core dependency management.

Changes in project dependencies:
- put proper dependency on h2o-core 2.5-SNAPSHOT
- put additional test dependency on h2o-core (type:test-jar), since H2O core tests
  contain H2O test harness (class water.TestUtil)
- removed dependency on water.TestUtil from project app code

Note: mvn test command still not working since H2O does not yet support
proper test execution via JUnitRunner (problem with classloading)