Correct travis image URL.

Readme.md

-# STACK 4.3 [![Build Status](https://travis-ci.org/github/maths/moodle-qtype_stack.svg?branch=master)](https://travis-ci.org/github/maths/moodle-qtype_stack/)
+# STACK 4.3 [![Build Status](https://api.travis-ci.org/maths/moodle-qtype_stack.svg?branch=master)](https://travis-ci.org/github/maths/moodle-qtype_stack/)
 
 STACK is an assessment system for mathematics, science and related disciplines.  STACK is a question type for the Moodle learning management system, and also the ILIAS learning management system.