Include ocaml-rpc 1.9.11 which fixes a bug

History of ocaml-rpc up to 1.9.11:

0f4d438 2017-03-03 (HEAD -> ppx2, tag: v1.9.11, github-lindig/ppx2, github-jonludlam/ppx2) Open Rpc module in is_string [Jon Ludlam]
d3ef65a 2017-03-03 (tag: v1.9.10) Launder the real type through an Rpc.t [Jon Ludlam]
a8738f8 2017-03-03 Fix marshalling of dictionaries [Jon Ludlam]
0fb57f1 2017-03-03 Add a test to check for correct marshalling of dictionaries [Jon Ludlam]