update to use upload/v4 endpoint

The `Content-Type` of `plain/text` was a mistake in v3. Fixed in v4.

Author: Steve Peak

lib/codecov.js

@@ -167,7 +167,7 @@ var sendToCodecovV3 = function(codecov_endpoint, query, upload_body, on_success,
   // Direct to S3
   request.post(
     {
-      url : urlgrey(codecov_endpoint + '/upload/v3').query(query).toString(),
+      url : urlgrey(codecov_endpoint + '/upload/v4').query(query).toString(),
       body : '',
       headers : {
         'Content-Type': 'text/plain',
@@ -184,7 +184,7 @@ var sendToCodecovV3 = function(codecov_endpoint, query, upload_body, on_success,
             url : result.split('\n')[1],
             body : upload_body,
             headers : {
-              'Content-Type': 'plain/text',
+              'Content-Type': 'text/plain',
               'x-amz-acl': 'public-read'
             }
           }, function(err, response, result){