Fix bookmark output on Mercurial 3.5.
Mercurial 3.5 renamed the `repo._bookmarkcurrent` attribute to
`_activebookmark`.
    
        | author | Kevin Bullock <kbullock@ringworld.org> | 
    
        | date | Fri, 31 Jul 2015 14:32:34 -0500 | 
    
    
        | parents | 869146b8b9fb | 
    
        | children | e90cf5cad060
34c0e34e7fe7 | 
    
        | branches/tags | (none) | 
    
        | files | prompt.py | 
Changes
    
--- a/prompt.py	Fri Jul 31 14:31:40 2015 -0500
+++ b/prompt.py	Fri Jul 31 14:32:34 2015 -0500
@@ -110,6 +110,8 @@
             book = getattr(repo, '_bookmarkcurrent', None)
         except KeyError:
             book = getattr(repo, '_bookmarkcurrent', None)
+        if book is None:
+            book = getattr(repo, '_activebookmark', None)
         if book:
             cur = repo['.'].node()
             if repo._bookmarks[book] == cur: