Default to using Mongo for caching

Stress testing shows that after the recent caching optimization there
seems to be no significant difference in performance when using Mongo
for caching as opposed to Redis. Redis is still supported but is now
optional.

Also removed hiredis dependency since as of node_redis 2.6 the default
parser is faster than hiredis.

package.json

@@ -19,13 +19,14 @@
     "bin": "bin"
   },
   "dependencies": {
-    "hiredis": "^0.5.0",
     "later": "^1.2.0",
     "libxmljs": "^0.18.4",
     "mongodb": "^2.2.24",
-    "redis": "^2.6.5",
     "seedrandom": "^2.4.2"
   },
+  "optionalDependencies": {
+    "redis": "^2.6.5"
+  },
   "devDependencies": {
     "coffeescript": "^1.12.4"
   },