[CI] initialization Frama-CI

But with clang 3.5 instead of 3.3

.gitlab-ci.yml

+Tests:
+  variables:
+    CURRENT: $CI_COMMIT_REF_NAME
+    DEFAULT: "master"
+    OCAML: "4_05"
+    FRAMA_CI_OPT: "--override frama-clang:$CI_COMMIT_REF_NAME,$CI_COMMIT_SHA"
+  script:
+   - nix/frama-ci.sh build -A frama-clang.tests
+  tags:
+   - nix

nix/default.nix

+# paramaterised derivation with dependencies injected (callPackage style)
+ { pkgs, stdenv, src ? ../., opam2nix,
+   ocaml_version ? "ocamlPackages_latest.ocaml", plugins ? { } }:
+
+plugins.helpers.simple_plugin
+   { inherit pkgs stdenv src opam2nix ocaml_version plugins;
+     name = "frama-clang";
+     deps = [ pkgs.clang_4 pkgs.llvm_4 ];
+     opamPackages = [ "camlp4" ];
+   }