Switch Travis to Xenial for more recent toolchain

This issue covers updating our Travis platform from Trusty (14.04) to Xenial (16.04). Trusty is using an old 4.8.4 gcc which makes it hard to reproduce toolchain-related failures locally.

Quoting @hgreving2304

4.8.4 is very old and it looks like it doesn't even support Sandybridge (1st gen AVX). Though it seems to support -mavx. But for anything w/ AVX (e.g. tests), a newer version would be better.