build: Move istanbul back to stable version

Follows-up 22d61f2, which moved istanbul from v0.2.4 to their 'harmony'
branch in order to support ES6 generator functions.

The branch has since then been merged and released as v0.3.9,
the branch hasn't been updated since.

Ref #127

Author: Krinkle

package.json

@@ -51,6 +51,6 @@
     "timekeeper": "^0.0.4"
   },
   "optionalDependencies": {
-    "istanbul": "https://github.com/gotwarlost/istanbul/tarball/harmony"
+    "istanbul": "0.3.9"
   }
 }