Use upgrade for Homu.

If upgrade isn't specified, it won't actually install the git revsision unless it has a newer version number.
servo · bors-servo · Mar 17, 2017 · Mar 17, 2017 · Mar 17, 2017 · 6f93524be720ecd16030e29fe2c05b17be8f5280
commit 6f93524be720ecd16030e29fe2c05b17be8f5280
diff --git a/homu/init.sls b/homu/init.sls
@@ -21,6 +21,7 @@ homu:
     - pkgs:
       - git+https://github.com/servo/homu@{{ homu.rev }}
       - toml == 0.9.1  # Please ensure this is in sync with requirements.txt
+    - upgrade: True
     - bin_env: /home/servo/homu/_venv
     - require:
       - virtualenv: homu