readme: Only display build status for master branch

README.md

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 [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/vapourismo/luwra)
-[![Build Status](https://travis-ci.org/vapourismo/luwra.svg)](https://travis-ci.org/vapourismo/luwra)
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+[![Build Status](https://travis-ci.org/vapourismo/luwra.svg?branch=master)](https://travis-ci.org/vapourismo/luwra)
 # Luwra
 A header-only C++ library which provides a Lua wrapper with minimal overhead.