When I run `./bin/celerity map` in the stable branch of phabricator, it aborts with an exception, when it reaches the sprint extension:
```
Rebuilding 2 resource source(s).
Rebuilding resource source "phabricator" (CelerityPhabricatorResources)...
Found 110 binary resources.
Found 362 text resources.
Found 9 packages.
Writing map "resources/celerity/map.php".
Rebuilding resource source "sprint" (CeleritySprintResources)...
Found 5 binary resources.
[2015-07-10 08:28:07] EXCEPTION: (Exception) CSS file 'phui-object-box.css' has unknown variable 'fontfamily'. at [<phabricator>/src/applications/celerity/CelerityResourceTransformer.php:334]
arcanist(head=stable, ref.master=999eb9376568, ref.stable=d54cb072facd), libphremoteuser(head=master, ref.master=1def4e2d7f07), phabricator(head=stable, ref.master=f0bf7a8e2621, ref.stable=2e82fb07cdc1), phutil(head=stable, ref.master=aa6cd8f7e5e5, ref.stable=83f09f6c5a03), sprint(head=production, ref.master=4912ecf41ef1, ref.production=13c2949b3777)
#0 CelerityResourceTransformer::replaceCSSVariable(array)
#1 preg_replace_callback(string, array, string) called at [<phabricator>/src/applications/celerity/CelerityResourceTransformer.php:161]
#2 CelerityResourceTransformer::replaceCSSVariables(string, string) called at [<phabricator>/src/applications/celerity/CelerityResourceTransformer.php:44]
#3 CelerityResourceTransformer::transformResource(string, string) called at [<phabricator>/src/applications/celerity/CelerityResourceMapGenerator.php:186]
#4 CelerityResourceMapGenerator::rebuildTextResources(CeleritySprintResources, CelerityResourceTransformer) called at [<phabricator>/src/applications/celerity/CelerityResourceMapGenerator.php:54]
#5 CelerityResourceMapGenerator::generate() called at [<phabricator>/src/applications/celerity/management/CelerityManagementMapWorkflow.php:47]
#6 CelerityManagementMapWorkflow::rebuildResources(CeleritySprintResources) called at [<phabricator>/src/applications/celerity/management/CelerityManagementMapWorkflow.php:24]
#7 CelerityManagementMapWorkflow::execute(PhutilArgumentParser) called at [<phutil>/src/parser/argument/PhutilArgumentParser.php:406]
#8 PhutilArgumentParser::parseWorkflowsFull(array) called at [<phutil>/src/parser/argument/PhutilArgumentParser.php:301]
#9 PhutilArgumentParser::parseWorkflows(array) called at [<phabricator>/scripts/setup/manage_celerity.php:21]
```