Make travis badge only report status of master branch

Like in #1313

README.md

@@ -1,13 +1,13 @@
 Learn about the Elm programming language at [elm-lang.org](http://elm-lang.org/).
 
-[![Build Status](https://travis-ci.org/elm-lang/elm-compiler.svg)](https://travis-ci.org/elm-lang/elm-compiler)
+[![Build Status](https://travis-ci.org/elm-lang/elm-compiler.svg?branch=master)](https://travis-ci.org/elm-lang/elm-compiler)
 
 ## Install
 
 Follow [these instructions][installer] to use Elm on your machine. Be sure to use
 the platform specific installers if you are on Mac or Windows. It's way easier!
 
- [installer]: https://github.com/elm-lang/elm-platform/blob/master/README.md#elm-platform 
+ [installer]: https://github.com/elm-lang/elm-platform/blob/master/README.md#elm-platform
 
 ## Build from source / Contribute
 
@@ -22,4 +22,4 @@ how the script works and what your workflow will be like.
 If you are stuck, email
 [the list](https://groups.google.com/forum/?fromgroups#!forum/elm-discuss)
 or ask a question in the
-[#Elm IRC channel](http://webchat.freenode.net/?channels=elm). 
+[#Elm IRC channel](http://webchat.freenode.net/?channels=elm).