README: Specify master branch for Travis CI badge

README.md

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 Citra Emulator
 ==============
-[![Travis CI Build Status](https://travis-ci.org/citra-emu/citra.svg)](https://travis-ci.org/citra-emu/citra)
+[![Travis CI Build Status](https://travis-ci.org/citra-emu/citra.svg?branch=master)](https://travis-ci.org/citra-emu/citra)
 [![AppVeyor CI Build Status](https://ci.appveyor.com/api/projects/status/sdf1o4kh3g1e68m9?svg=true)](https://ci.appveyor.com/project/bunnei/citra)
 
 Citra is an experimental open-source Nintendo 3DS emulator/debugger written in C++. It is written with portability in mind, with builds actively maintained for Windows, Linux and OS X. Citra only emulates a subset of 3DS hardware, and therefore is generally only useful for running/debugging homebrew applications. At this time, Citra is even able to boot several commercial games! Most of these do not run to a playable state, but we are working every day to advance the project forward.