allow AWS creds to be specified in the environment, so that codeship can push to S3 after a build success

grunt/config/s3.js

@@ -3,7 +3,6 @@ try {
   config = require('../../.aws-config.json');
 } catch (e) {}
 
-
 module.exports = {
   upload_archives: {
     upload: [
@@ -13,8 +12,8 @@ module.exports = {
       }
     ],
     options: {
-      key: config.key,
-      secret: config.secret,
+      key: config.key || process.env.AWS_KEY,
+      secret: config.secret || process.env.AWS_SECRET,
       bucket: 'download.elasticsearch.org',
       access: 'public-read',
       headers: {