Only use os.cpus() and filter out core like physical-cpu-count did (#142)

* Only use os.cpus() and filter out core like physical-cpu-count did

The problem is with heroku and some CI, the physical-cpu-count linux branch requires a binary and access not provided by these environments.

* using physical-cpu-count and defaulting to os.cpus()

* get the actual length, should be good

Author: reel

src/WorkerFarm.js

@@ -1,4 +1,5 @@
 const {EventEmitter} = require('events');
+const os = require('os');
 const Farm = require('worker-farm/lib/farm');
 const promisify = require('./utils/promisify');
 
@@ -8,7 +9,7 @@ class WorkerFarm extends Farm {
   constructor(options) {
     let opts = {
       autoStart: true,
-      maxConcurrentWorkers: require('physical-cpu-count')
+      maxConcurrentWorkers: getNumWorkers(),
     };
 
     super(opts, require.resolve('./worker'));
@@ -86,4 +87,14 @@ for (let key in EventEmitter.prototype) {
   WorkerFarm.prototype[key] = EventEmitter.prototype[key];
 }
 
+function getNumWorkers() {
+  let cores;
+  try {
+    cores = require('physical-cpu-count');
+  } catch (err) {
+    cores = os.cpus().length;
+  }
+  return cores || 1;
+}
+
 module.exports = WorkerFarm;