Upgrade to Sphinx 1.5.3

Read the Docs is now using the latest version of Sphinx, so upgrade our
recommended version too.

As of Sphinx 1.4, index entries are 5-tuples instead of 4-tuples. Update
the Cretonne Sphinx domain to generate the new 5-tuples.

Since we're over this compatibility bump, there's no reason to recommend
a specific Sphinx version, so just go back to 'current'.

docs/cton_domain.py

@@ -65,7 +65,8 @@ class CtonObject(ObjectDescription):
         indextext = self.get_index_text(name)
         if indextext:
             self.indexnode['entries'].append(('single', indextext,
-                                              targetname, ''))
+                                              targetname, '', None))
+
 
 # Type variables are indicated as %T.
 typevar = re.compile('(\%[A-Z])')
@@ -113,6 +114,7 @@ class CtonType(CtonObject):
     def get_index_text(self, name):
         return name + ' (IL type)'
 
+
 sep_equal = re.compile('\s*=\s*')
 sep_comma = re.compile('\s*,\s*')