Add Travis CI badge

Now that Travis CI is running our tests, add a badge to the README to
show off the status of the tests.

Change-Id: I7440a9193c45bbc5da517a733c296522ac38513d
Reviewed-on: https://gerrit.causes.com/23739
Tested-by: Joe Lencioni <joe.lencioni@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>