Fix build status icon

README.md

 Problem Builder XBlock
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-[![Build Status](https://travis-ci.org/open-craft/xblock-problem-builder.svg?branch=master)](https://travis-ci.org/open-craft/xblock-problem-builder)
+[![Build Status](https://travis-ci.org/open-craft/problem-builder.svg?branch=master)](https://travis-ci.org/open-craft/problem-builder)
 
 This XBlock allows creation of questions of various types and simulating the
 workflow of real-life mentoring, within an edX course.