mate-accessibility-compiz: bugfix: fix upgrade from older release.

This upgrade was broken due to:

1) Wrong upgrade function name: I have renamed from upgrade_v0_6 to
upgrade_v0_7 to match the caller from base_install.py

2) wrong username related to internal changes when moving to the Python
pwd library that has changed the structure: I have switched to the new
wrapper get_username

Signed-off-by: Cyril Brulebois <cyril@debamax.com>