fix(package): update lru-cache to version 5.1.0

Closes #41

Author: greenkeeper[bot]

package.json

@@ -35,7 +35,7 @@
     "harp-minify": "^0.4.0",
     "less": "^2.5.3",
     "lodash": "^4.16.6",
-    "lru-cache": "^4.0.0",
+    "lru-cache": "^5.1.0",
     "marked": "^0.3.5",
     "node-sass": "^4.1.0",
     "postcss": "^6.0.0",