use /usr/local/qgis as default

The rationale is that so twe dont need to update the script on every new qgis release.
If we developers are installing various qgis versions il say:
/usr/local/qgis-2.2
/usr/local/qgis-2.4
/usr/local/qgis-2.6

we can also do a symlink /usr/local/qgis to our latest qgis
ln -s /usr/local/qgis-2.6 /usr/local/qgis