Upgrade Jython to version 2.7b3 and xlrd to version 0.9.3.

Jython's development has resumed, and the bug which prevented us to upgrade to 2.7b1 (see issue #33) has been fixed. Tests seem to pass now (and I'm happily able to use networkX 1.9 and stuff like "with" statements in my scripts), but it would be a good idea to run a few more scripts from the osb just to make sure we're not breaking anything.