Changed Ubuntu version to 12.04

Author: mmasdeu

setup.py

@@ -9,7 +9,7 @@
 from distutils.command import build as build_module
 
 # Obtain the different Sage versions
-def get_all_version_names(mirror_url, idx = None, distribution = 'Ubuntu_16.04-x86_64'):
+def get_all_version_names(mirror_url, idx = None, distribution = 'Ubuntu_12.04-x86_64'):
     if idx is None:
         idx = 0
     else: