readme: fixed header in response to github/markup#1022
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-<img src="doc/criterion-title.png" height="96" alt="Criterion Logo" />
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+<h1><img src="doc/criterion-title.png" height="96" alt="Criterion Logo" /></h1>
[![Unix Build Status](https://img.shields.io/travis/Snaipe/Criterion/bleeding.svg?label=linux)](https://travis-ci.org/Snaipe/Criterion)
[![Windows Build Status](https://img.shields.io/appveyor/ci/Snaipe/Criterion/bleeding.svg?label=windows)](https://ci.appveyor.com/project/Snaipe/Criterion/branch/bleeding)
