Add initial compile cache spec

src/compile-cache.coffee

@@ -7,6 +7,8 @@ to5 = require './6to5'
 # package install so that the first package load in Atom doesn't have to
 # compile anything.
 exports.addPathToCache = (filePath, atomHome) ->
+  atomHome ?= process.env.ATOM_HOME
+
   cacheDir = path.join(atomHome, 'compile-cache')
   # Use separate compile cache when sudo'ing as root to avoid permission issues
   if process.env.USER is 'root' and process.env.SUDO_USER and process.env.SUDO_USER isnt process.env.USER