Update Travis CI build status badge - show master branch status

[ci skip]

Fixes #264.

Previously this seemed to be showing the status of the last build, even
if the last build was for a branch or PR.

README.md

@@ -1,4 +1,4 @@
-## Mocha [![build status](https://secure.travis-ci.org/freerange/mocha.png)](https://secure.travis-ci.org/freerange/mocha) [![Gem Version](https://badge.fury.io/rb/mocha.png)](http://badge.fury.io/rb/mocha) [![OpenCollective](https://opencollective.com/mocha/backers/badge.svg)](#backers) [![OpenCollective](https://opencollective.com/mocha/sponsors/badge.svg)](#sponsors)
+## Mocha [![Build Status](https://travis-ci.org/freerange/mocha.svg?branch=master)](https://travis-ci.org/freerange/mocha) [![Gem Version](https://badge.fury.io/rb/mocha.png)](http://badge.fury.io/rb/mocha) [![OpenCollective](https://opencollective.com/mocha/backers/badge.svg)](#backers) [![OpenCollective](https://opencollective.com/mocha/sponsors/badge.svg)](#sponsors)
 
 
 ### Description