change default API rev from v1beta14 to v1beta15
change default API rev from v1beta14 to v1beta15

googlearchive/compute-appengine-demo-suite-python,googlearchive/compute-appengine-demo-suite-python,jonparrott/compute-appengine-demo-suite-python,gdgjodhpur/compute-appengine-demo-suite-python,gdgjodhpur/compute-appengine-demo-suite-python,jonparrott/compute-appengine-demo-suite-python,jonparrott/compute-appengine-demo-suite-python,jonparrott/compute-appengine-demo-suite-python,googlearchive/compute-appengine-demo-suite-python,googlearchive/compute-appengine-demo-suite-python,gdgjodhpur/compute-appengine-demo-suite-python,jonparrott/compute-appengine-demo-suite-python,googlearchive/compute-appengine-demo-suite-python,gdgjodhpur/compute-appengine-demo-suite-python,gdgjodhpur/compute-appengine-demo-suite-python