use EPHEMERAL_STORAGE instead of HEROKU env

afaik you can have persistent storage on heroku, at least via addons
and heroku is probably not the only environment where one has a temporary file system

app.json

@@ -10,7 +10,7 @@
     ],
     "website": "https://crafatar.com/",
     "env": {
-      "HEROKU": "true",
+      "EPHEMERAL_STORAGE": "true",
       "BUILDPACK_URL": "https://github.com/mojodna/heroku-buildpack-multi.git#build-env"
     },
     "addons": [

lib/cache.js

@@ -8,11 +8,11 @@ var fs = require("fs");
 var redis = null;
 
 // sets up redis connection
-// flushes redis when running on heroku (files aren't kept between pushes)
+// flushes redis when using ephemeral storage (e.g. Heroku)
 function connect_redis() {
   logging.log("connecting to redis...");
   // parse redis env
-  var redis_env = (process.env.REDISCLOUD_URL || process.env.REDIS_URL);
+  var redis_env = process.env.REDISCLOUD_URL || process.env.REDIS_URL;
   var redis_url = redis_env ? url.parse(redis_env) : {};
   redis_url.port = redis_url.port || 6379;
   redis_url.hostname = redis_url.hostname || "localhost";
@@ -23,15 +23,15 @@ function connect_redis() {
   }
   redis.on("ready", function() {
     logging.log("Redis connection established.");
-    if (process.env.HEROKU) {
-      logging.log("Running on heroku, flushing redis");
+    if (process.env.EPHEMERAL_STORAGE) {
+      logging.log("Storage is ephemeral, flushing redis");
       redis.flushall();
     }
   });
-  redis.on("error", function (err) {
+  redis.on("error", function(err) {
     logging.error(err);
   });
-  redis.on("end", function () {
+  redis.on("end", function() {
     logging.warn("Redis connection lost!");
   });
 }
@@ -67,15 +67,14 @@ exp.get_redis = function() {
 // updates the redis instance's server_info object
 // callback: error, info object
 exp.info = function(callback) {
-  redis.info(function (err, res) {
-
+  redis.info(function(err, res) {
     // parse the info command and store it in redis.server_info
 
     // this code block was taken from mranney/node_redis#on_info_cmd
     // http://git.io/LBUNbg
     var lines = res.toString().split("\r\n");
     var obj = {};
-    lines.forEach(function (line) {
+    lines.forEach(function(line) {
       var parts = line.split(":");
       if (parts[1]) {
         obj[parts[0]] = parts[1];
@@ -99,7 +98,7 @@ exp.info = function(callback) {
 // callback: error
 exp.update_timestamp = function(rid, userId, hash, temp, callback) {
   logging.debug(rid, "updating cache timestamp");
-  var sub = temp ? (config.caching.local - 60) : 0;
+  var sub = temp ? config.caching.local - 60 : 0;
   var time = Date.now() - sub;
   // store userId in lower case if not null
   userId = userId && userId.toLowerCase();
@@ -117,8 +116,8 @@ exp.save_hash = function(rid, userId, skin_hash, cape_hash, callback) {
   logging.debug(rid, "caching skin:" + skin_hash + " cape:" + cape_hash);
   var time = Date.now();
   // store shorter null byte instead of "null"
-  skin_hash = (skin_hash === null ? "" : skin_hash);
-  cape_hash = (cape_hash === null ? "" : cape_hash);
+  skin_hash = skin_hash === null ? "" : skin_hash;
+  cape_hash = cape_hash === null ? "" : cape_hash;
   // store userId in lower case if not null
   userId = userId && userId.toLowerCase();
   if (skin_hash === undefined) {