Use master branch for readme's travis build badge

Now that travis is passing again on master, we can provide the master build badge instead of cheating and showing the cpan one.

README.md

@@ -1,6 +1,6 @@
 # Selenium::Remote::Driver
 
-[![Build Status](https://travis-ci.org/gempesaw/Selenium-Remote-Driver.svg?branch=cpan)](https://travis-ci.org/gempesaw/Selenium-Remote-Driver)
+[![Build Status](https://travis-ci.org/gempesaw/Selenium-Remote-Driver.svg?branch=master)](https://travis-ci.org/gempesaw/Selenium-Remote-Driver)
 
 [Selenium WebDriver][wd] is a test tool that allows you to write
 automated web application UI tests in any programming language against