Add redis-server and [travis_ignore]

These changes allow the install.sh file to do a full clean install on a new machine. The travis build environment already contains a redis-server package that is a more recent version than that which is available from the Ubuntu 12.04 packages. Prior attempts ( Dimagi pull request #4112) caused the stock redis-server on the travis build to have to be restarted. This change allows travis to ignore the redis-server apt-get package, but fully installs on a new machine.