upgrade lru-cache module to avoid performance regression with node 4.2

Fix performance regression on Node 4x #55
https://github.com/isaacs/node-lru-cache/pull/55

services/spelling/package.json

@@ -13,7 +13,7 @@
     "coffee-script": "^1.9.1",
     "express": "^4.12.1",
     "logger-sharelatex": "git+https://github.com/sharelatex/logger-sharelatex.git#v1.1.0",
-    "lru-cache": "^2.5.0",
+    "lru-cache": "^4.0.0",
     "metrics-sharelatex": "git+https://github.com/sharelatex/metrics-sharelatex.git#v1.3.0",
     "mongojs": "1.3.0",
     "node-statsd": "0.0.3",