readme: Make Travis build status badge reflect the state of master

README.md

 libgit2 - the Git linkable library
 ==================================
 
-[![Build Status](https://secure.travis-ci.org/libgit2/libgit2.png?branch=development)](http://travis-ci.org/libgit2/libgit2)
+[![Travis Build Status](https://secure.travis-ci.org/libgit2/libgit2.png?branch=master)](http://travis-ci.org/libgit2/libgit2)
 [![Coverity Scan Build Status](https://scan.coverity.com/projects/639/badge.svg)](https://scan.coverity.com/projects/639)
 
 `libgit2` is a portable, pure C implementation of the Git core methods