Drop support for Node 6

.travis.yml

@@ -4,7 +4,6 @@ cache:
   directories:
     - node_modules
 node_js:
-  - '6'
   - '8'
   - '10'
 sudo: false

README.md

@@ -48,9 +48,6 @@ const stripe = Stripe('sk_test_...');
 //…
 ```
 
-On older versions of Node, you can use [promises](#using-promises)
-or [callbacks](#using-callbacks) instead of `async`/`await`.
-
 ## Initialize with config object
 
 The package can be initialized with several options:
@@ -135,28 +132,6 @@ stripe.customers
   });
 ```
 
-### Using callbacks
-
-On versions of Node.js prior to v7.9:
-
-```js
-var stripe = require('stripe')('sk_test_...');
-
-stripe.customers.create(
-  {
-    email: '[email protected]',
-  },
-  function(err, customer) {
-    if (err) {
-      // Deal with an error (will be `null` if no error occurred).
-    }
-
-    // Do something with created customer object
-    console.log(customer.id);
-  }
-);
-```
-
 ### Configuring Timeout
 
 Request timeout is configurable (the default is Node's default of 120 seconds):

package.json

@@ -22,7 +22,7 @@
   },
   "bugs:": "https://github.com/stripe/stripe-node/issues",
   "engines": {
-    "node": "^6 || ^8.1 || >=10.*"
+    "node": "^8.1 || >=10.*"
   },
   "main": "lib/stripe.js",
   "devDependencies": {