Avoid using deprecated Buffer API

Use Buffer.from when it's available instead of 'new Bufer(string)'

Fixes: npm#60
Refs: https://nodejs.org/api/deprecations.html#deprecations_dep0005_buffer_constructor

lib/collect.js

@@ -12,7 +12,15 @@ function collect (stream) {
   stream.on('end', save)
   var buf = []
   function save (b) {
-    if (typeof b === 'string') b = new Buffer(b)
+    if (typeof b === 'string') {
+      if (Buffer.from && Buffer.from !== Uint8Array.from) {
+        b = Buffer.from(b)
+      } else {
+        // Outdated Node.js versions (<4.5.0 or 5.x <5.10)
+        // b is already type-checked to be a string just above
+        b = new Buffer(b)
+      }
+    }
     if (Buffer.isBuffer(b) && !b.length) return
     buf.push(b)
   }