Updates build badge to point to the master branch

Author: arqex

README.md

@@ -1,6 +1,6 @@
 # react-datetime
 
-[![Build Status](https://secure.travis-ci.org/YouCanBookMe/react-datetime.svg)](https://travis-ci.org/YouCanBookMe/react-datetime)
+[![Build Status](https://secure.travis-ci.org/YouCanBookMe/react-datetime.svg)](https://travis-ci.org/YouCanBookMe/react-datetime.svg?branch=master)
 [![npm version](https://badge.fury.io/js/react-datetime.svg)](http://badge.fury.io/js/react-datetime)
 
 A date and time picker in the same React.js component. It can be used as a datepicker, timepicker or both at the same time. It is **highly customizable** and it even allows to edit date's milliseconds.