Riv can't find implicit link targets (section references)

Hi and thanks for the great plugin!

After updating Vim and Riv.Vim Riv cannot find implicit section references.
Example:
```
grep
====
.. _grep1:

bla bla bla

`grep1`_ 
`grep`_
```
After pressing <Enter> on `grep1`_ it works.
After pressing <Enter> on `grep`_ I got an error: "Riv: Could not find the link targets"
Folding seems to work.