Set the context class loader before running Jython scripts

This corresponds to the commit 2a1d5292(Refresh_JRuby_Scripts: set the
context class loader correctly) which fixed the same issue for JRuby
scripts.

Noticed by Jean-Yves Tinevez in bug 657.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>