[travis] Install krb5-greet-client where available
[travis] Install krb5-greet-client where available

This causes greet_clien tests to pass on Travis.

Closes #57.

frozencemetery/python-gssapi,pythongssapi/python-gssapi,frozencemetery/python-gssapi,pythongssapi/python-gssapi,frozencemetery/python-gssapi,pythongssapi/python-gssapi