Set correct Python executable path

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>

--- python-ipython-2.1.0/setupbase.py	2014-05-21 15:19:12.000000000 -0400
+++ python-ipython-2.1.0-new/setupbase.py	2014-07-23 23:34:32.892030423 -0400
@@ -394,7 +394,7 @@ class build_scripts_entrypt(build_script
 
             mod, func = entrypt.split(':')
             with open(outfile, 'w') as f:
-                f.write(script_src.format(executable=sys.executable,
+                f.write(script_src.format(executable='/usr/bin/env python',
                                           mod=mod, func=func))
 
         return outfiles, outfiles