switch Travis Linux build to xenial

[lnihlen]

Purpose and motivation

An attempt to fix #124 by moving the build server to Linux Xenial.

Implementation

Swtich the build server to Xenial, fix whatever breaks.

Types of changes

• Bug fix

Status

• [ ] This PR is ready for review
• [ ] Unit tests added
• [ ] Unit tests are passing

[lnihlen]

Whoops, meant to merge with develop