no longer test on or support Coq 8.5

.gitlab-ci.yml

 image: ralfjung/opam-ci:latest
 
-iris-coq8.5.3:
-  tags:
-  - coq
-  script:
-  # prepare
-  - . build/opam-ci.sh 'coq 8.5.3' 'coq-mathcomp-ssreflect 1.6.1'
-  # build
-  - 'time make -j8 TIMED=y 2>&1 | tee build-log.txt'
-  - 'if fgrep Axiom build-log.txt >/dev/null; then exit 1; fi'
-  - 'cat build-log.txt | egrep "[a-zA-Z0-9_/-]+ \(user: [0-9]" | tee build-time.txt'
-  - 'if (( RANDOM % 10 == 0 )); then make validate; fi'
-  cache:
-    key: "coq8.5.3"
-    paths:
-    - opamroot/
-  only:
-  - master
-  - ci
-  - timing
-  artifacts:
-    paths:
-    - build-time.txt
-
 iris-coq8.6:
   tags:
   - coq

README.md

@@ -6,7 +6,7 @@ This is the Coq development of the [Iris Project](http://iris-project.org).
 
 This version is known to compile with:
 
- - Coq 8.5pl3 / 8.6
+ - Coq 8.6
  - Ssreflect 1.6.1
 
 The easiest way to install the correct versions of the dependencies is through
@@ -14,6 +14,9 @@ opam.  Once you got opam set up, just run `make build-dep` to install the right
 versions of the dependencies.  When the dependencies change, just run `make
 build-dep` again.
 
+If you need to work with Coq 8.5, please check out the
+[iris-3.0 branch](https://gitlab.mpi-sws.org/FP/iris-coq/tree/iris-3.0).
+
 ## Building Instructions
 
 Run `make` to build the full development.