Upgrade to Xenial and ICU4C 64.2 in Travis CI
Created by: sciyoshi
This also removes Python 2.6 and 3.3 from the test matrix and updates the download URL for ICU4c. Note that it uncovered an issue in Python 2.7, where the script code for surrogates is Zyyy instead of Zzzz. Not sure why this only seems to happen on Python 2: