Bump nan to 2.2.0

This should fix building for node 5.8.0+

Author: John Haley

package.json

@@ -51,7 +51,7 @@
     "lcov-result-merger": "~1.0.2",
     "lodash": "~3.10.1",
     "mocha": "~2.3.4",
-    "nan": "~2.0.9",
+    "nan": "^2.2.0",
     "node-gyp": "~3.0.3",
     "nw-gyp": "~0.12.4"
   },