[travis] Install krb5-greet-client where available

This causes greet_clien tests to pass on Travis.

Closes #57.

Author: frozencemetery

.travis.install.sh

@@ -13,5 +13,6 @@ fi
 
 sudo apt-get update -q
 DEBIAN_FRONTEND=noninteractive sudo apt-get install -y krb5-user krb5-kdc krb5-admin-server libkrb5-dev krb5-multidev
+DEBIAN_FRONTEND=noninteractive sudo apt-get -y install krb5-greet-client || true
 pip install --install-option='--no-cython-compile' cython
 pip install -r test-requirements.txt