CI: use macOS Mojave

PullRequest: truffleruby/539

Author: eregon

ci.jsonnet

@@ -40,6 +40,10 @@ local part_definitions = {
         PATH: std.join(":", self.path + ["$PATH"]),
       },
 
+      packages+: {
+        "pip:ninja_syntax": "==1.7.2", # Required by NFI and mx
+      },
+
       setup+: [
         # We don't want to proxy any internet access
         ["unset", "ANT_OPTS", "FTP_PROXY", "ftp_proxy", "GRADLE_OPTS",
@@ -367,7 +371,7 @@ local part_definitions = {
       ],
       "$.run.specs":: { test_spec_options: ["-GdarwinCI"] },
       "$.cap":: {
-        normal_machine: ["darwin_sierra", "amd64"],
+        normal_machine: ["darwin_mojave", "amd64"],
       },
       environment+: {
         LANG: "en_US.UTF-8",