readme: remove gittip

README.md

@@ -7,7 +7,6 @@
 [![Dependency Status][david-image]][david-url]
 [![License][license-image]][license-url]
 [![Downloads][downloads-image]][downloads-url]
-[![Gittip][gittip-image]][gittip-url]
 
 Run multiple functions in parallel with concurrency.
 Only supports functions that return promises.
@@ -49,5 +48,3 @@ batch.then( => );
 [license-url]: LICENSE
 [downloads-image]: http://img.shields.io/npm/dm/batch-then.svg?style=flat-square
 [downloads-url]: https://npmjs.org/package/batch-then
-[gittip-image]: https://img.shields.io/gratipay/jonathanong.svg?style=flat-square
-[gittip-url]: https://gratipay.com/jonathanong/