[CI] Bump to llvm 7

nix/default.nix

@@ -5,9 +5,9 @@
 plugins.helpers.simple_plugin
    { inherit pkgs stdenv src opam2nix ocaml_version plugins;
      name = "frama-clang";
-     deps = [ pkgs.llvmPackages_4.clang-unwrapped pkgs.llvm_4 pkgs.gnused ];
+     deps = [ pkgs.llvmPackages_7.clang-unwrapped pkgs.llvm_7 pkgs.gnused ];
      opamPackages = [ "camlp4" ];
      postPatch = ''
-     sed -i Makefile.config.in -e "s&@CLANG_INCDIR@&${pkgs.llvmPackages_4.clang-unwrapped}/include&"
+     sed -i Makefile.config.in -e "s&@CLANG_INCDIR@&${pkgs.llvmPackages_7.clang-unwrapped}/include&"
      '';
    }