doen't use path.join for urls

 * the path module is for filesystem path. It is plaform agnostic
   and will add backslashs on windows. That is not the expected
   behavior.

lib/s3.js

@@ -37,7 +37,7 @@ module.exports = CoreObject.extend({
     var prefix       = options.prefix;
     var manifestPath = options.manifestPath;
     if (manifestPath) {
-      var key = path.join(prefix, manifestPath);
+      var key = [prefix, manifestPath].join('/');
       plugin.log('Downloading manifest for differential deploy from `' + key + '`...', { verbose: true });
       return new Promise(function(resolve, reject){
         var params = { Bucket: options.bucket, Key: key};
@@ -81,7 +81,7 @@ module.exports = CoreObject.extend({
       var data        = fs.readFileSync(basePath);
       var contentType = mime.lookup(basePath);
       var encoding    = mime.charsets.lookup(contentType);
-      var key         = path.join(prefix, filePath);
+      var key         = [prefix, filePath].join('/');
       var isGzipped   = gzippedFilePaths.indexOf(filePath) !== -1;
 
       if (encoding) {

tests/unit/index-nodetest.js

@@ -226,7 +226,7 @@ describe('s3 plugin', function() {
         name: 's3'
       });
 
-      context.gzippedFiles = ['app.css'];
+      context.gzippedFiles = ['/app.css'];
       var assertionCount = 0;
       context.uploadClient = null;
       context.s3Client = {