XD-1766 Fixing failing tcptofile test

 - Added --outputType=text/plain option to tcp source as
   that is now required.

More info can be found from here: https://github.com/spring-projects/spring-xd/commit/9928025ae64b0aa01fdef0783a8a6ac4cebf4f3d

This is already mentioned in tcp source docs that text/plain is needed in a case like this.