(minor improvement) Use variable for SparkR assembly versioning in Makefile; ignore folders and files generated in unit tests to avoid accidental checkins

pkg/src/Makefile
* Makefile: factor out SPARKR_VERSION to reduce potential copy&paste error...
* cp & rm called with -f in build/clean phase

* .gitignore includes checkpoints and unit test log generated by run-tests.sh
style nit - can you add a space after the `=` to match the other lines ?