Add Travis build status to readme.
Add Travis build status to readme.

walshb/pyavroc,Byhiras/pyavroc,walshb/pyavroc,crs4/pyavroc,crs4/pyavroc,walshb/pyavroc,Byhiras/pyavroc,Byhiras/pyavroc,walshb/pyavroc,crs4/pyavroc,Byhiras/pyavroc