Require at least Sphinx 1.4 since the format of index tuples changed.

cranelift/docs/conf.py

@@ -29,7 +29,7 @@ sys.path.insert(0, os.path.abspath('../lib/codegen/meta'))
 
 # If your documentation needs a minimal Sphinx version, state it here.
 #
-# needs_sphinx = '1.0'
+needs_sphinx = '1.4'
 
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom