[SW-389] Print extended JAR  task every time when extendJar is called (#240)
[SW-389] Print extended JAR  task every time when extendJar is called (#240)

* [SW-389] Print extended JAR  task every time when extendJar is called

* Minor gradle update.

Includes:
  - replace use of File by `file`
  - get rid of File.separator - the JVM File used by Gradle is already platform agnostic. However,
  we still need to use File.separator if we are constructing path for external programs (e.g., python)
  See: https://docs.gradle.org/current/javadoc/org/gradle/api/file/FileTreeElement.html#getPath()

* Add task to clean cached H2O Jars

h2oai/sparkling-water,h2oai/sparkling-water,h2oai/sparkling-water,h2oai/sparkling-water