Bump Iris, fix build.

opam

@@ -9,5 +9,5 @@ build: [make "-j%{jobs}%"]
 install: [make "install"]
 remove: ["rm" "-rf" "%{lib}%/coq/user-contrib/iris_atomic"]
 depends: [
-  "coq-iris" { (= "dev.2017-12-18.1") | (= "dev") }
+  "coq-iris" { (= "dev.2018-02-21.1") | (= "dev") }
 ]

theories/peritem.v

@@ -5,7 +5,7 @@ From iris.proofmode Require Import tactics.
 From iris.algebra Require Import frac auth gmap csum.
 From iris.base_logic Require Import big_op.
 From iris_atomic Require Export treiber misc.
-From iris.base_logic.lib Require Import invariants namespaces.
+From iris.base_logic.lib Require Import invariants.
 
 Section defs.
   Context `{heapG Σ} (N: namespace).