Add Travis badge for core integration branch

Author: jamo

README.markdown

@@ -1,5 +1,7 @@
 # Test My Code NetBeans plugin #
 
+[![Build Status](https://travis-ci.org/testmycode/tmc-netbeans.svg?branch=core_integration)](https://travis-ci.org/testmycode/tmc-netbeans)
+
 This is a NetBeans plugin for the [Test My Code server](https://github.com/testmycode/tmc-server). It allows downloading, testing and submitting exercises directly from the IDE.
 
 ## Developer setup ##