Spark installation improved by sourcing the `spark-config.sh` in the `before-notebook.d` hook that is run by `start.sh`. It permits to add automatically the right Py4J dependency version in the `PYTHONPATH`. So it is not needed anymore to set this variable at build time.
Documentation describing the installation of a custom Spark version modified to remove this step. Also updated to install the latest `2.x` Spark version.
`test_pyspark` fixed (was always OK before that).