readme: Changed travis badge to reflect bleeding builds
<img src="doc/criterion-title.png" height="96" alt="Criterion Logo" />
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-[![Unix Build Status](https://img.shields.io/travis/Snaipe/Criterion/master.svg?label=linux)](https://travis-ci.org/Snaipe/Criterion)
+[![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)
[![Coverage Status](https://img.shields.io/codecov/c/github/Snaipe/Criterion/bleeding.svg)](https://codecov.io/github/Snaipe/Criterion?branch=bleeding)
[![License](https://img.shields.io/badge/license-MIT-blue.svg)](https://github.com/Snaipe/Criterion/blob/master/LICENSE)
