Fix some broken tests for Python 3

Remove python2 dependencies in .travis.yml and add python3-paste.
Convert t_daemon.py and jsonwalker.py to python3.  csjon has no
python3 version, so replace it with python's built-in JSON module.

python3-pyrad isn't available for Trusty, so krad and OTP tests are
currently not exercised by Travis.

[ghudson@mit.edu: squashed commits; edited commit message]

https://github.com/krb5/krb5/commit/d1fb3551c0dff5c3e6555b31fcbf04ff04d577fe
Author: Robbie Harwood <rharwood@redhat.com>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: d1fb3551c0dff5c3e6555b31fcbf04ff04d577fe
Branch: master
 .travis.yml              |    2 +-
 src/lib/krad/t_daemon.py |    2 +-
 src/tests/jsonwalker.py  |   16 +++++-----------
 3 files changed, 7 insertions(+), 13 deletions(-)