Update the travis build image to display the build result from master…

… branch only

This will prevent someone to think that Paperclip test suite for master branch is currently failing, as the build was failed on a WIP branch, not the main branch.

http://travis-ci.org/#!/thoughtbot/paperclip/builds.

README.md

@@ -1,4 +1,4 @@
-# Paperclip [![Build Status](https://secure.travis-ci.org/thoughtbot/paperclip.png)](http://travis-ci.org/thoughtbot/paperclip)
+# Paperclip [![Build Status](https://secure.travis-ci.org/thoughtbot/paperclip.png?branch=master)](http://travis-ci.org/thoughtbot/paperclip)
 
 Paperclip is intended as an easy file attachment library for ActiveRecord. The
 intent behind it was to keep setup as easy as possible and to treat files as